> In what context would you want to write out a slur against someone

There are other contexts than using a slur against someone. For example, describing an instance where a slur was used against you personally, explaining historical contexts of the slur, and so on. The existence of a slur does not mean that it's only ever going to be used in cases where it's being thrown at someone else

Right but in the example you use you could write sl*r or sl-r instead of slur and anyone reading would understand you and you would get past the filter.

No, you couldn't. For example, if you were teaching someone who's learning English about which words to avoid, they wouldn't know which words you're talking about since you're not actually using them