Lots of banks, lenders and insurers will not take on people with credit problems. There are a few factors.
One being if you are unable to manage your own finances then how are you in a position to mange other peoples finances, although this usually applies to banks and lenders.
Another is the publicity - if some unscrupulous member of the public or press decide that they have an agenda against a certain organisation and wish to make it known publicly that they employ people who have had troubles handling their financial affairs.
They may be checking up to see if you have been honest on your application form when asked about this. The non-disclosure on an application form is seen as a bigger problem than the IVA.
Some businesses insurers (that provide indemnity insurance) will specify certain catergories of people that they can't employ or that they won't cover if they are employed. These traditionally include criminals with unspent convictions but may also include people who have been bankrupt, entered into an IVA, etc.
Some businesses apply a credit score system, as a bank would before offering you a loan, to clarify whether a person will be offered employment.
Some businesses just won't trust someone who has money troubles into a position where they can abuse their position to make financial gain.
There are a whole host of reasons why they may want to carry out a check but being honest with them up front is the only policy. Ask them why they want it and explain to them why you have a worry.