Occasionally, spammers show up on PPCG and post spam, including stuff like promotion of (probably fake) products, (probably fake) offers to transfer money into your account (after you give them $500 dollars), etc.
Upon seeing this, it might make sense to downvote, flag, and delete. However, if everyone does that, then the answer (or question) will be deleted before a Six Flag Combo can be processed.
How should we treat spam posts?
If three people vote to delete it before six spam flags get through, then the post will have to be undeleted before it can continue to be flagged. If six flags get through, that post is deleted and locked by Community (and gets its content shoved into the revision history), which will also remove 100 reputation from that user (assuming they had any, which they almost always don't) and eventually ban their account and/or that IP address.
If people delete the post, the spammer will be free to continue, which clutters up our beloved site (citation-needed) and is in general, extremely annoying to deal with.
If everyone instead flags it as spam and downvotes it (a spam flag is also an automatic downvote from Community), it will be deleted by Community which can also help stop the spammer from continuing.
So overall, don't vote to delete spam; flag it as spam, maybe post a link to chat (along with a note to not delete it), and be on your way :)
In addition to HyperNeutrino's excellent answer:
If you see a spam post, make sure to check in Charcoal HQ and see if it's been reported. If not, report it with !!/report <url> - you'll be helping a group of volunteers enhance a system that automatically scans every post on the SE network and tries to figure out if it is spam based on previous spam post patterns. Spammers are constantly getting more sophisticated to get around spam filters, so do your part to fight back against them!