What is the benefit of working through an Umbrella Company?

As a contractor you may be able to offset business expenses against your taxable pay to help reduce your deductions, providing these expenses are incurred wholly, necessarily and exclusively in the course of your assignment duties & subject to you meeting certain criteria (see below for further detail).
The amount of expense that you have incurred in carrying out your duties will be paid free of PAYE & NI. This means that your take home pay is increased because you are paying less Tax and NI. It is important to remember that unlike when you are employed we do not reimburse you for those expenses; we simply process them according to the HMRC guidelines for temporary places of work.