Connecting flight to Phuket

While going to immigration at BKK, I have seen signs "Transfer to Phuket / Chiang Mai..". What are these?

Being a tourist, I have to cross immigration but have a flight to Phuket (on a separate ticket), can I go to these signs instead of "Arrival or Immigration"?

Will that save time? Also, is 2 hour 30 minutes enough for transfer from International to domestic flight (separate ticket). I usually see answer to keep 3 hours.