Relay and exit node IPs aren't private, so admins will often collect them and just block them en masse. This causes problems, because a lot of that same IP space will often be shared with things like pool.ntp.org nodes.
By default Tor doesn't make any attempt to disguise the fact it's Tor traffic. Bridges are unlisted relays, which should work for IP blocking, while Plugable Transports are made to evade censorship-focused traffic analysis (i.e., they will prevent the adversary from detecting and blocking Tor connections as a whole, but not prevent a dedicated adversary from figuring out "has this client been using Tor?").
