Not just for open bases, but also for “compactified” bases that are used to “farm” gems from attacking players that think “oh, looks easy, let’s rush in there!” and then have to scroll/resurrect… It’s always nice to log back in after a day and have dozens of gems waiting for you!
Moreover, some people also like to change their bases a bit from time to time to make it harder for others that attack them every now and then - if you get familiar with a base, it is much easier to beat it, thus a few different layouts are good to keep your base challenging and surprising.
Also, comparing different base layouts (say A,B,C) would be a lot easier as you could test them more easily without having to recreate A/B/C all the time…
And, last but not least, sometimes I just want to try out a certain tower combination or test some troop combinations for attacking - having a kind of “test design” to switch to would be kinda cool
So, lots of uses for a “save defense” functionality
Then, one important addition/remark : Saving the defense waves together with the base layout would be very helpful then! Different base layout require different wave troops (e.g. some more path overlap is good for gargoyles, frosters or mortars; a path with few/none overlap works better with ogres; etc etc…), and currently it is a quite troublesome process to change of your wave troops… even more time consuming and painstaking than changing the layout of path and towers.