I totally agree that this is a huge issue. Especially if you don't do much coding)

These vendors are probably solving the problem of letting you automate your billing and invoicing process without developing your own system.

As you said earlier, there's already a lot of Ask HN about this. So a solution does exist.

I take your point that on the other hand it may require a lot of input on my part into an already existing program.

For example, if you were going from project to project as a developer - you would probably be doing your own system. And if as a project manager - you would look for an existing solution on the market, hoping to reduce development costs))