A lot of domains have that feature btw. Gmail's specific feature is with the dots functioning as catch-all [1] (though Facebook apparently has the very same feature).
[1] http://www.slate.com/blogs/future_tense/2013/08/01/dots_in_g...
Also, it's very easy to mechanically identify all such users because of the + symbol, which, if you are trying to prevent your real email address from revealed means it's not that useful...
Regarding the latter, when they email you directly without the + you can be very strict. You could even apply whitelisting.
It's very useful
In other words, there's a big difference between using ikea@ and saastrial1@, saastrial2@, saastrial3@,.. and so on to keep signing up for trials with the same SaaS provider.
[1] -- https://www.fastmail.com/help/receive/addressing.html