The main purpose of it is to be a base for next development phase/release as you rightly said.
Before the next release or next phase, the applicable documents like for example Test Cases are baselined which means that for the previous release, the base lined test cases are the final ones and no changes can be done later.
Whatever the changes we do to that test cases fall under next release.
You can think of baseline as a base for building mean to say that a base line is like a reference document or a deviced plan of work untill any thing is changed in that.
to be simple the future work is based on the baseline document and if we want to make changes to the baseline that should be through a formal change request, after all the modifications are done if we think it is enough for the future work we will baseline that document untill the next release.