Why?
1. Skip the Queue - When my uncle told me about checking in online last December, I tried it on a Jetstar flight to Singapore and was quite glad that I did. A dedicated web check-in counter is available at the airport which allowed me to skip the queue and save time.
2. Choose Your Seat - On my recent trip to Bangkok via Thai airways, I forgot to do so and I spent almost half an hour waiting. What's worse is that I ended up in the middle seat squeezed between two big guys. I forgot to mention that another benefit of web check-in is that you get to choose your seat.
The night before my trip back to Singapore, I made sure to use the online check-in facility. Good thing I did because I saw that I was once again assigned a middle seat. I was able to change my seat selection and check myself in which just two simple clicks.
3. Pre-Flight Time - I also noticed that instead of two hours prior, you can show up at the airport just an hour before your scheduled flight.

The photo above is the reminder I got stating that I have to be at check-in counter at least 60 minutes prior to the scheduled departure time.
This is what greeted me when I arrived at the airport at 5 in the morning.


The crazy long queues I skipped and was out of there in less than 10 minutes.
I normally fly budget and good thing is that they also offer this service.
1. Air Asia: It's actually better to check-in online because you avoid paying counter check-in fees which are imposed on domestic flights, they also have printing terminals at some airports to allow you to print the boarding pass yourself
Air Asia: http://www.airasia.com/my/en/myflights/mciguides.page
2. Cebu Pacific: If I'm not mistaken, they offered this service for a fee before but have since removed it.
Cebu Pacific: http://www.cebupacificair.com/travel-services/web-check-in.html
3. Jetstar: http://www.jetstar.com/sg/en/planning-and-booking/checking-in/web-check-in
4. Tiger's not offering the service yet but from the site, it should be available soon.
Tiger: http://www.tigerairways.com/sg/en/webcheckin_guide.php
5. PAL's not offering the service.