When I worked in an office, there was a canteen where we could get free tea and coffee, or water. A tradesman working in your home has no access to a canteen. Ok, so they can (and my husband does) take water, lunch, etc with them. That's fine. But what if they want a hot cup of coffee? The options are that you can offer, and they can drink it whilst they work and get on with the job. Or they can get in the ute, drive to a cafe, have a cup of coffee, drive back, then get back into working. It would be much quicker to just offer to put the kettle on. I mean, how much does a teabag and some milk or a teaspoon of coffee cost anyway?
This is assuming of course that they are not on the clock, like a previous poster found to her detriment. My husband is paid for the job, regardless of how long it takes, so offering a cup of coffee just means he is done faster with less need to leave the house.
That is of course not mentioning the general politeness of offering refreshments to someone in my home. Personally I would be mortified if anyone, guest or worker, in my home had to ask for a cup of tea.