Because for a lot of people needs must.
It's easy to say charge £40 an hour if you are established, have enough work anyway, have your own house, have a partner working, are comfortable, etc.
If you're in digs, have a couple of young kids, struggling to find work, £20 an hour is a lifesaver.
It's also easy to say, but if you charge £40 an hour you won't be struggling, but you have to get in the position where turning down those £20 an hour jobs is an option 1st.
EDIT: Sorry for the long... thought/post...
I disagree, time is money at the end of the day... travelling from one job to another doesn't cover itself... as I said previously, if you have 10 x 1 hour jobs in one day at the very least you're looking at a minimum 13 hour day... you're losing out straight away on £60.00 with the travelling...
Then you've got to look at costs between jobs, for instance if you're doing man and van work and you need to drop off at a council tip, there's costs involved there... you have to factor that into the job, until you get to the tip you have no idea how much it's going to cost...
In essence, my hourly rate is based upon what it would cost me to run a "man and van" type business that will not cost me anything... for instance, I've worked out that over a 10 hour day, 52 weeks per year for me to run a van, cover insurances, taxes, NI, "some" diesel costs cover for holidays if I ever took any would cost me on average £22.00 per hour... therefore if I was to sell out my services at £40.00 per hour I am covering everything within reason and ensuring that I am also earning a small but relatively affordable to live on crust.
The fact that I am working as a Locksmith for a company means that I don't need to do this yet, but my weekend work that I do covers me in case I needed too. As I said previously, I charged £200 per day for a Saturday and a Sunday, I only did 7 hours per day... but in that 7 hours per day I grafted... I didn't stop for anything, I got stuck in... had I stopped for anything the days would have been longer... to me, it's all about perspective and relativity - do you want to work for peanuts? do you want to be treated like a monkey? or do you want to be treated with the respect that you deserve as a hard working man or woman and earn what you deserve?
It's irrelevant to me as to whether you're just starting up or not, if you have the experience and knowledge (as the OP has stated he has) then he deserves to earn respectively of that knowledge and experience...
If you're a labourer, you're a labourer... If it's all you know, its all you will do... but I can tell you now, there's labourer's out there that are earning £1000 a week because they graft and get stuck in.
Then there's people who are just breaking even, just covering their bills because they're stuck in a dead end job fighting to stay on top... My job covers more than I need and some... the fact that my wife has open fingers and money runs through them like water... is another story altogether

- my passion is woodworking... at some point it might begin to bear fruits of my labour and enable me to work within my passion... but until then, I'll keep on as I am
