Why do they need to be checked?

If you are having an alteration or addition made to your electrical installation, your electrician must first check (as well as other things), that the earthing and bonding arrangements you have are up-to the required standard. This is because the safety of any new work (however small), will depend on the earthing and bonding (as does the safety of your existing installation).
What is Earthing?
If there is a fault in your electrical installation you could get an electric shock if you touch a metal part that has become live due to the fault. This is because the electricity may use your body as a path from that metal part to earth. Earthing is used to protect you from an electric shock. It does this by providing a path (a protective conductor) for a fault current to flow to earth. It also causes the protective device – a circuit-breaker, fuse or residual current device (RCD) – to switch off the electricity supply to the circuit that has the fault. For example, if a machine (or domestic appliance) has a fault, the fault current flows to earth through the protective conductor. The relevant circuit-breaker, fuse or RCD in the consumer unit switches off the electrical supply to the machine . The machine is now safe from causing an electric shock to anyone who touches it.
What is Bonding?
Bonding is used to reduce the risk of electric shock to anyone who may touch two separate metal parts when there is a fault somewhere in the electricity supply or electrical installation. By connecting together the particular metal parts with bonding conductors, bonding reduces the voltage there The two types of bonding normally used are main bonding and supplementary bonding. Main bonding is always needed. You may need supplementary bonding for rooms containing a bath or shower, except where all circuits in the room are RCD protected and the main bonding is up to the required standard.