Very useful for new founders, although as you quite rightly point out if you pay a half-decent accountancy firm to handle your company finances they should be sorting most of this out for you!
For example, my contracting company took out a policy for the Professional Indemnity cover, but it also comes with some Public Liability and sufficient Employer's Liability cover to meet statutory obligations if we take on employees.
I don't know whether this works out cheaper than finding separate policies for exactly what you need, so obviously please do your homework if you're in a position of needing this type of cover.
EL is the only compulsory insurance which applies to all private sector employers, although in certain industries you will have additional requirements (e.g. conveyancers have to purchase professional indemnity insurance).