First..
What is the objective of 2? What is the purpose of this +spam email if you will delete all email to it?
It is because you were required to fill in a form or questionnaire, to get access to some content or "reward".
The site and its requirement for email are asking not because they really needed it, but because they are going to use it to contact you, send you additional email, or suspiciously they may harvest and sell the addresses.
You don't want to have to manually create multiple real email addresses separate from your main one, nor do you want to completely fake it in case you need the first message (generally you'd tag as spam so you could go find the first message from said sites) SO, using the +keyword, you make it possible to receive messages, give a site or form an email address that it demanded of you, regardless of their stated "we won't spam you" lies, and then you have the ability to get the "benefit" that required this information.
Example: Want to get papa johns or pizza hut or any large chain pizza with a discount coupon? .. Search google for coupon sites, some require a registration, not to email you the code, but just so you could SEE the code on their site.
Solution, you give them your real email address with a +spam.coupon.site.name on it, you know where the spam came from, you know you wouldn't want their emails, and can set the rule to automatically put into spam or delete it, etc.
Second .. do all domains support it? A quick test suggests no. I tried sending from gmail to my personal domain, and the message is rejected. I'm sure there are options for many of the open email servers out there, to reconfigure to accept +keyword messages, but it would depend on you having that ability to configure it for your domain.
1additional note about "don't see advantage ... two different addresss" correct and its a disadvantage to manually create a new address. If on the fly you could add +spam.fake.site@gmail without having to do any back end manual sign-up tasks, you save yourself time. .... My suggestion below as answer .. is add +spam.site.name.com@gmail.. so you have the keyword "spam" in there, and then put the WWW site name in for "site.name.com" as a reminder of which site sold your address to others. – TG2 – 2016-09-24T06:24:25.233