It’s my understanding that the Last flag acts as a }, i.e., it closes a BLOCK of rewrite conditions and rules so, as you would use } to close a block of PHP statements, so would you use [L] to close a “block” of mod_rewrite statements.
That said, it’s best to use [L] at the end of EACH RewriteRule unless it’s to chain to another rule - [R=301,L] is highly recommended.