There are both good and bad reasons why the shop comes back with a laundry list of parts. A lot of the cost of these repairs is labor, so you hate to drain the coolant and tear everything off the front of your engine to replace a thermostat for example, and then have to tear it all apart again in six months or a year to replace a water pump. I do all my own work on my four BMWs and I have a policy of - "If it comes off for any reason and it is questionable at all, replace it, because I do NOT want to do this again soon." Pelican parts sells parts in kits for guys like me. They look at every job, consider the parts you will have to remove to do the job, plus the normal wear parts that are easy to replace while you are in there, and they put them all in a kit with a group price. It is a big $$ bill for parts, but you spend a lot fewer Saturdays in your garage under your car.
More importantly, on an 8-year-old car, all the plastic and rubber parts are getting shrunk and brittle, so any of them could fail at any time. The shop knows that if they spend $500 of your money replacing cooling system parts, and a month later something else fails and the cooling system starts leaking again, you will blame them for 'not fixing it right' and want them to fix it at their own expense, even if the new leak is a part they did not touch. Hence, they want to replace all related parts on your tab, to make sure they don't see you again next month. This liability motive is OK sometimes (like with cooling systems) but sometimes is really expensive for you (electrical systems) which is why you save a lot of money if you do your own work.