In The Force Awakens, we see Poe Dameron and Finn use a TIE fighter to escape the First Order. Later, when Rey and Finn try to escape Jakku, we see that the stormtroopers chasing them in TIE fighters wear this back stormtrooper spacesuit. But if the TIE fighters have life support systems, why is it that the pilots of TIEs always wear them?
I'd imagine they wear them for the same reason as everyone travelling on a Soyuz wears a Sokol space suit. The real-world suits are designed for an environment where there is a pressure hull and life support, but since that can go wrong it also has its built-in pressurisation and life support as a precaution. If you lose people to depressurisation, you adopt extra measures to prevent it, and unlike larger Star Wars vessels with bulkhead doors, a TIE has no other way to deal with a hull breach. Being able to survive a non-catastrophic breach is a tactical gain, as well as not having to recruit and train another pilot. It's more surprising that the pilots of other small craft don't wear them.
In lots of movies set on earth, you see people in cars which have airbags and seatbelts. There doesn't need to be an explanation in the screenplay for such mundane precautions, I wouldn't expect one in the Star Wars canon, however Wraith Leader found "Though TIE vessels have minimal, perfunctory oxygen scrubbers and pressurized atmospheric seals, TIE pilots routinely wear full flight gear as a precaution." confirming that it's a precaution.