Learn Russian: Эта причина важная, так как мы хотим понять проблему. - This reason is important, since we want to understand the problem.

In this sentence, they mean the same thing and you could use either. так как translates nicely to "since" or "as" and sounds just a bit more formal and logical than the standard потому что (because). Also, while you can easily start a new sentence with Так как... ("Since we want to..."), you generally shouldn't start a sentence with Потому что.

Do I always need a comma before так как?

Yes. Russian punctuation is much stricter than English. You must place a comma before conjunctions that introduce a subordinate clause (a part of the sentence with its own subject and verb). Since так как connects the main idea to the reason, the comma is mandatory.

Could I use the short adjective важна instead of the full form важная?

Yes, you could say Эта причина важна. Both are correct, but they have a slightly different feel. The full form важная is very common in everyday speech and describes the reason as inherently important. The short form важна sounds a bit more formal and emphasizes that it is important right now for our specific situation.

Why use the perfective verb понять (to understand) instead of the imperfective понимать?