Your sentence is ambiguous and has at least two possible interpretations:
- Understanding A is useful for many purposes, but you're going to write about B.
- There are many things useful for understanding B, but you're going to write about A.
This is most likely not a problem, because it's probably obvious from context which you mean. In particular, if you were just talking about A, readers will interpret it as sense (1), while if you were just talking about B, readers will interpret it as sense (2). If you were recently talking about both A and B, then you should consider restructuring your writing to separate them and use this sentence as a transition.
Secondly, you do not need the extra clause. You can recast the sentence as follows:
The understanding of A is also/___ particularly/mainly/especially relevant to a better understanding of the B.
This is easier to read because the adverbs don't interrupt the main clause.
- particularly - A/B has many uses/prerequisites, but you're going to talk about B/A, because that's what your paper/article/etc. is about.
- especially - As before, but you're talking about it because it's a substantial use or prerequisite and not just because you feel like it.
- mainly - As before, but you're talking about it because it's the most important one.