.. to know how people feel.
We, as members of society, need information when we choose who to befriend, who to do business with, and who to avoid. If people are not free to express themselves, we cannot make good choices.
If you're the type to avoid humor in bad taste, and you find this person offensive, you should be fighting this. Otherwise, at some point, you may find yourself emotionally attached to someone who feels this way, only to find out after the fact. When people bare their soul in a public forum, without fear of repercussion, you can observe and make decisions on how you want to interact with them in private. You can decide ahead of time.
If speaking one's mind is potentially illegal, much important information becomes unavailable as people will be unwilling to speak their minds. You cannot know someone is a racist, or someone is opposed to religious influence, or someone is against liberal governance, or someone has a problem with war. You can't know if they're neo-nazis, and you can't know if they believe politicians should be hanged for war crimes.
You may enter into relationships with these people only to find out much later that they feel a certain way.
The more deeply offended you are by speech, the more you should fight for it to be open and free.