On the other hand if there's training a permie wants that the employer doesn't want to pay for they can pay full whack for it out of their post tax income. As a contractor I can reclaim the VAT on that training course and write it off against my tax bill.
Being based from home so that every mile you drive to work each day is expensed through the company. PCs, laptops, tablets etc are all easy to pay for with pre-tax income because they're trivial to show business use for.
It's not just that we get paid more than a permie, it's that we can deduct so much stuff from our tax bill that they can't, you can end up earning more per hour and paying way less tax.