Actually none from experience. It's there for a really good reason.
Say you want to visit www.hahaha.com but your network admin is a rotten bastard, and this does happen a lot in corporate networks, well what happens is they set up a DNS record of www.hahaha.com.myevilcorporation.local and set your default domain suffix to myevilcorporation.local.
When you hit www.hahaha.com you don't go to www.hahaha.com; you go to some internal site. The browser tends not to give you any clues about this so you just carry on without knowing.
If you visit www.hahaha.com. (note the trailing dot) then you go to www.hahaha.com. as that dot is the root of the DNS heirarchy and myevilcorporation.local never ever resolves.
Of course this throws all sorts of warnings if you are using SSL but for the average user and site, that isn't necessarily true. Hell we did it years ago and deployed an internal CA cert to all our workstations so it looks like a ton of our corporate sites are actually internet-based and SSL covered but are on internal networks only with private certs.
I suspect that this is less of an issue these days and browsers have protection against this but I wouldn't be 100% sure of it.
Public service announcement: It's best to use "example.com" (or .net, .org, etc) for this sort of thing, because there's no real server there to accidentally send traffic to or alter the search rankings of.
It's not usually that big a deal, but every so often someone will paste the wrong thing or an automated process will misinterpret something. It's better if this happens to a nonexistent domain.
Say you want to visit www.hahaha.com but your network admin is a rotten bastard, and this does happen a lot in corporate networks, well what happens is they set up a DNS record of www.hahaha.com.myevilcorporation.local and set your default domain suffix to myevilcorporation.local.
When you hit www.hahaha.com you don't go to www.hahaha.com; you go to some internal site. The browser tends not to give you any clues about this so you just carry on without knowing.
If you visit www.hahaha.com. (note the trailing dot) then you go to www.hahaha.com. as that dot is the root of the DNS heirarchy and myevilcorporation.local never ever resolves.
Of course this throws all sorts of warnings if you are using SSL but for the average user and site, that isn't necessarily true. Hell we did it years ago and deployed an internal CA cert to all our workstations so it looks like a ton of our corporate sites are actually internet-based and SSL covered but are on internal networks only with private certs.
I suspect that this is less of an issue these days and browsers have protection against this but I wouldn't be 100% sure of it.
So in conclusion, it's really important.