hey guys I have a question in regards to Starfire 055 from Rebirth.

Her power reads

Attack STARBOLTS: Energy Explosion. Starfire may use it at no cost instead of a range attack while using Hypersonic Speed. (You still halve her range.)

What was the purpose of the "at no cost" in that sentence? Just thought it was weird that it is there and maybe someone can explain why.

"At no cost" has to be there to make the effect work properly. You can't give actions (Energy Explosion here) while another action is already resolving (Hypersonic Speed) without that phrasing. Check the rule book on page 8.

uhm.... thank you very much!

Since I am home now and for completeness:

Obviously WK could word the effect to do whatever they want, but they are trying to keep everything working technically correct with the rules as written.