1) Yes, I would say it's necessary. You never know when there's a little crack in the heater or something else has failed, just waiting for a path to ground. You don't want to be that path to ground.
2) Absolutely yes there is danger. Breakers only trip at >15A current, and they are SLOW. It takes much less than 15A to kill you and it will happen before a breaker will trip.
3) GFCI are usually required by code anywhere there is water. They detect a current imbalance between line (120V) and neutral (power return which is attached to earth ground at the breaker box). When a GFCI detects less current flowing in the neutral (return path) than through the line (120V) conductor, it cuts off power.
This means that if you touch the hot (120V) line and become a conductor to ground, it detects that there is no return current on the neutral line, and trips. They are much faster and much more sensitive than circuit breakers; even slight imbalances (milliamps) will cause them to trip. Circuit breakers provide almost no form of electrocution prevention; GFCI's provide some.
4) Install a GFCI outlet on your aquarium, and turn gear off before sticking a hand in. Take precautions so that sprays, drips, etc do not get into wiring extensions, etc
From the US Consumer Product Safety Commission:
In the home's wiring system, the GFCI constantly monitors electricity flowing in a circuit, to sense any loss of current. If the current flowing through the circuit differs by a small amount from that returning, the GFCI quickly switches off power to that circuit. The GFCI interrupts power faster than a blink of an eye to prevent a lethal dose of electricity. You may receive a painful shock, but you should not be electrocuted or receive a serious shock injury.
Here's how it may work in your house.. Suppose a bare wire inside an appliance touches the metal case. The case is then charged with electricity. If you touch the appliance with one hand while the other hand is touching a grounded metal object, like a water faucet, you will receive a shock. If the appliance is plugged into an outlet protected by a GFCI, the power will be shut off before a fatal shock would occur.
Take it from an electronics engineer that's been hit by everything from line voltage up to 45kV plasma discharges: GFCI's can be a lifesaver, and you are foolish if you don't have one on your tank after you've read this.