Is it useless to play ranked games since the season is near over?

The season itself is over already.

However it is still good to keep on at ranked, your current rank will carry over to pre-season and whatever rank you end pre-season on will influence your placement MMR. Effectively the higher rank you when Season 5 starts the better people you play against, giving you more of a chance to be placed higher right off the bat. As a tip, be sure to get used to the pre-season changes before tackling ranked or you may find yourself surprised in a bad way.

Ranked should still be played with the intention of moving up because it will help you in your placement if you have a higher mmr.

You can increase your MMR by winning which can improve your placement for next season and helps with your LP gains/losses.