It's 15 bucks well invested. I also thought I don't need that and then they pulled me out for interrogation at Suvarnabhumi. Ticket was screened back an forth and copies taken by them.
yes, better be prepared. There are many providers. I just shared my own experience, can be different for you but I realized they hate to see border runs and extension stamps in your passport.
Outward flight not needed for visa application, but be prepared the IO pulls you out at Suvarnabhumi for interrogation. They didn't like the previous extension stamps in my passport and scrutinized my onward ticket, even put the data into the system.