We as humans have some important senses which we need for living. One of the main ones is our sight. We just want to see what is around us, what is in front of us, where we go, and so on. The same applies underwater. The water is a different environment and we would like to see what is there.
The space there is 3D. We can move in all directions which looks like seeing is even more important. Hearing is much less here even if the sound travels a bigger distance because under the water we can not recognize from which direction it comes, and it is just noise for us. I guess you know this feeling when you are snorkeling, you are under the surface and suddenly you hear some boat coming. It's just noise everywhere around and you need to look up and scan the surface to avoid any collision.
Our eyes are designed to see in the air. The water has different densities and changes how the light travels which causes us to see blurry in the water. The human eye is designed to focus light onto the retina, but underwater light bends twice and it causes the image to be formed somewhere behind the retina.
The solution is a mask. As soon as we put it on our face, our eyes are in the air and we can see as if we are above the surface. Objects just seem to be a bit bigger and a bit closer. The index of refraction of the water is 1,33, and of the air is 1. It means the angle from which the rays of light reach your eyes is larger than in the air. But ok, it doesn't really matter and if you are freediving regularly, you will just get used to it. You have a mask now, you can see what is around, where you swim, you can observe the reef and watch sea creatures, take pictures, or hunt. With the mask, you can see as clearly as on the land.
But nothing is that simple. Mask is an airspace and we have to equalize it. We can talk about semi-flexible airspace because the mask is soft. Mask has a silicone skirt which allows it to change shape a bit while under the pressure. Another thing is they are low volume. It simply means that we don't need that much air to equalize it and it doesn't cost us so much effort in comparison with bigger scuba diving masks which are not designed for freediving. But it is an airspace and you have to equalize it anyway. And it costs some air, some focus, you need to know how to do it, it can cause some troubles and the result can be the end of your descent.
The physics is clear. With depth, the ambient pressure increases and compresses the air. It is described by Boyle's law. The bigger the pressure, the smaller the volume and it is inversely proportional. It means we have to equalize all airspace we have in our head such as the middle ears and sinuses. There are a few techniques we can use. In the first meters, it is usually the Frenzel technique or hands-free (if you know how to do it) and with greater depth mouthfill or Frenzel with reverse packing. That's about inner airspaces and that's what freedivers are usually very focused on during the dive itself and what is a big part of freediving training. And because we want to make our dive as easy as possible and eliminate mistakes or oxygen (air) consumption we are usually diving without a mask. The reason is very simple. We are freediving on the line, we know where we go, and we know (or trust) there is nothing we can hit. It's just the line with some stopper in the end. We want to be as streamlined as possible and we are not looking down. We don't care what is there, there is just a depth we want to go. We want to have perfect body position and just sink. Eliminate everything we don't need.
If you look at any competition you would see most of the freedivers diving just with the noseclip or with noseclip and goggles. Some are using fluid goggles which are upgraded by correcting lenses. If you wear them under the water the correction lenses make our sight sharp. You can see the line, tags, lanyard if it gets tangled and so on. You can even read data from your computer if you need to. The main reason why I'm using fluid goggles is to eliminate the stream of water passing across my eyes, because it makes me feel like I have to swallow. And the faster I go, the more I need it. I always like to dive just with the noseclip and use as few things as possible, but this just makes my dives way more comfortable. This is also a reason why you can see some freedivers using just normal goggles without correction lenses. They don't need to see better, they just want to keep their eyes covered. Diving with noseclip has a huge benefit because we don't need to equalize the mask and we have both of our hands-free. We don't need to pinch our nose and we can place them in the position we want.
Anyway, I would recommend training with the mask as well. You get better control of equalization. Better control of the soft palate and better technique of reverse packing. If you train only with the noseclip, one day you can just realize you struggle when diving deep with a mask. And to have this skill is very important especially for deep safety and more if you are doing deep safety with the scooter. Get the mask that fits you best, buy a noseclip, try some fluid goggles and go and train with all these tools. It's fun and it makes your freediving better.