Why would you want to send them from a website when the reply will come to your phone and leave you with half of each conversation in 2 seperate places?
One answer from a business perspective would be so that someone else can email out "texts" on your behalf e.g someone in an administritive role. A typical scenario might be where you want to collect responses from different groups or classes of people (i.e. you are only interested in their responses), whereas several different style messages may be required to be sent out to different types of people to achieve the same end.
In summary, you may wish to spend more time working ON your business whilst your administrator is left to concentrate on working IN the business.
Tmobile offer this and it's rather good. Could GiffGaff offer this too. It's a lot quicker to type a SMS from a computer than a mobile (well it is for me !)
I have bought a voucher and when I try to enter thisvoucher it tells me the number is not valid than you tiamoira