I've found that doing sessions without fins helps you get your kicking technique back on track and also builds up some strength your legs. Mainly because you have to make the extra effort to gain distance.

And also it helps to get your overall technique flowing smoothly so you can benefit from it (ie, your arms and legs are working together - you're not moving just because your arms are pulling you through the water, you're moving because both your arms and legs are gripping the water and pulling you through).