Why in all video tutorials people create textures with sizes like 1024x1024 or 2048x2048 or like that? I understand that this numbers related with powers of two, but why I cant use texture 800x800 for example? It will be works slower or what?
By ensuring the texture dimensions are a power of two, the graphics pipeline can take advantage of optimizations related to efficiencies in working with powers of two. For example, it can be (and absolutely was several years back before we had dedicated GPUs and extremely clever optimizing compilers) faster to divide and multiply by powers of two. Working in powers of two also simplified operations within the pipeline, such as computation and usage of mipmaps (a number that is a power of two will always divide evenly in half, which means you don't have to deal with scenarios where you must round your mipmap dimensions up or down)
This is a quote by stackexchange user "user1430", who answered the same question here.
Hope this helped :)