The explanations I have heard include:
- Helping to keep weight centered.
Helping to keep weight over the balls of the feet.
Helping to keep you off your heels.
At one time, Aikido was practiced outside so sliding the feet allowed a person to watch for obstructions without looking down.
The first three make a lot of sense. I don't know if the third one is anything more than an interesting bit of historical factoid.