I think they have decided to rename it since it now has improved features.
Now you can still measure the distance between points, but now you have also the option to calculate it along a way, by "sticking" to roads and tracks.
If you want to know what's the distance you will travel, you don't need to point multiple times to follow every single bend, just click the start and finish points, and the line will be drawn along a calculated route between
Hence the name "Plan a route".
Personally, I find that very comfortable (and of course the old option to measure point-by-point is still available)
Max