Here is a link to a topic I posted a few months ago where I outlined why forum signatures are awful and should be avoided. Signature in user post - #4 by Nyx1819.
When considering the use case outline by @UrgentCactus709, there is another reason; Signatures tend to be a global setting, so if you add more RAM to your computer the signature showing your system specs is now out of date in your previous topics.
The possible method I suggested above in post #5, you would get around this issue as the system specs at the time the topic was created would be baked into the post.