If you are genuinely unsure and interested, just ask him to explain what he will do (obviously there are ways of asking which will insult anybody and you won't get any information, and ways which will help build an understanding between customer and engineer).
Although the boiler is going in the same kind of location and doing a boilers job, its going to have all its pipes (water, gas, flue for exhaust gases) and wires (power, controls) in different places. Its probably cheaper than if the engineer had to bring all those services to a 'new' place for your boiler, but still lots of work and in some ways more difficult because there will things already in the way - i.e. he'll have to work around what is there, moving things, filling in old holes etc. Oh and I guess he'll be taking your old boiler away as well - he's probably paying for waste disposal.
