I can only speak for the UK, but for tax & legal reasons any contractor ought to be working through a company.

Most find it efficient to run a small business of their own (an accountant will handle 99% of the paperwork) but others - for example those who don't expect to be a contractor for long - can join an "umbrella company" operated by an accountant. Then they invoice via the umbrella company and most of the money passes back to them. Generally this has a higher cost & less scope for reducing the tax burden, but has the benefit of avoiding the long-term commitment of running your own company.