Back in the day, when slots first appeared, the gameplay was quite crude. A player would need to insert a coin, pull the lever, and that would initiate a single spin of the reels. Once that spin was over, you'd have to repeat the entire process again, which was a bit tiresome at times.
This isn't to say that slots weren't popular among players, but it was in everyone's interest to make this process smoother, so developers started coming up with various improvements, finally creating slots with autoplay feature.

From the player’s perspective, the autoplay feature is convenient because it gets rid of the need to constantly click or press that spin button, which can actually make your hand hurt if you’re in for a long session. For casinos, this is advantageous because players will get to play more spins during the same period of time.
An important thing to understand is that slots play exactly the same whether you spin manually or use autoplay, so there is really no downside to slots with autoplay feature. However, if you belong to the group of players who like to enjoy every spin and love to take your sweet time between every spin, there’s absolutely nothing wrong with that, either. Autoplay or not, what really counts is how much fun you have playing, and that’s all that matters.