You don’t have to apply the mirror modifier. The result will be mirrored UVs (and texture). Sometimes this is preferable, especially lo-poly games. It’s a way to save texture space at the cost of noticeable symmetry in the texture.
If you apply the mirror modifier you can unwrap UVs asymmetrically. Usually this is preferable for higher-res projects where you want the option to paint textures uniquely on both sides of the model.
