Most medium to large scale software projects end up being modified at some point during the development process, as new business requirements or unexpected problems arise. Having invested time and money already, you are unlikely to want to switch suppliers now.
Your supplier knows this, so they are in a strong position to inflate additional costs and stretch timescales. This is especially likely if they gave you a low quote to win the business in the first place, knowing that there was room in the original contract or spec to wiggle later.
IT Intermediaries can take full management of your projects to control costs and timescales - or simply be on hand to advise on specific issues when you need specialist input.