to connect two closely related clauses without coordinating conjunction
to connect two closely related clauses with a conjunctive adverb
Explanation:
The two independent clauses are closely related and were merged into a single sentence; the semicolon was necessary because there was no coordinating conjunction used (for, and, nor, but, or, yet, so) and a conjunctive adverb (however) took that place. Only coordinating conjunctions can link independent clauses without the need for a semicolon.
