Mixing loud
Mixing loud is a useful technique when mixing! It is commonly taught to mix at lower levels. However, if you were to create a quiet mix (e.g. around -14 LUFS) and send that to a mastering engineer, it is very likely your balances between tracks will not be what you had originally mixed due to compression and limiting. Effectively, what you had mixed will not sound at all the same.
When you mix at something like -8 LUFS, you can really hear what’s going on. A change of a dB is significant!