As has been said already, constant email addresses are the main benefit.
I have used
123 Reg for several years and found the service to be fine.
Costs are minimal - if all you want is email and web space forwarding (use the free webspace provided by your ISP) then is only costs
0.09 pence per year (+ £2.50 one-time registration payment) for a .co.uk domain. If you want webspace or a .net/.com/.org name then the prices are a bit more but still reasonable.
I have several .net domains as they have a bit more cachet than the common .co.uk
