Step 1: Make topic Step 2: Ignored bully harasses you in topic Step 3: Other people respond to harassment Step 4: Topic is disrupted
Even if you can't see quoted posts because you've enabled that option (and there's plenty of reason not to, since even if you generally don't care to read what somebody has to say, taking every post that quotes them out of a conversation can be really disruptive, especially when people multi-quote), you can still see posts that respond without quoting them, which is usually enough to infer what they're saying, pluys they've still derailed your topic. Blocking them addresses the issue much more simply and with fewer unwanted side effects than any other option can, with the only downside being that they can't see anything of yours that they might want to interact with more legitimately (which there's little reason to care about).
Ignore may be enough for your needs, and that's fine, but you are not everyone and other people sometimes do actually need Block.