When buying new tires (tyres), many shops will recommend performing "TPMS service" on each tire.
What is this service, and is it recommended? Or is it just a way for shops to pad their revenue?
TL;DR: Essentially, TPMS service is replacing the wheel units on a vehicle with dTMPS where the units have either failed or where the battery is low enough that it will likely run out before the next time the tires would need to be demounted. For cars without TMPS or cars with iTMPS the service is unnecessary. If it is being offered "blindly" (without checking if the car uses dTMPS or the condition of the batteries, then yes it is probably just a way to pad revenue. But if it is being done in response to actual failure or anticipated failure, it is a good idea.
TPMS stands for Tire Pressure Monitoring System, hopefully any shop that is recommending the service is actually checking to confirm that the vehicle actually has TPMS capability and that the system requires service. In the US TPMS became mandatory for light (under 10,000 pounds) vehicles in 2007. In the EU TPMS became mandatory for all new vehicle models in 2012 and for all passenger cars in 2014. Korea followed suit requiring TPMS for new models in 2103 and for all mandated vehicles in 2014. Indonesia, Israel, Malaysia, the Philippines, Russia, and Turkey have also made TPMS mandatory.
There are two variants on TPMS:
Direct (dTPMS) systems, where a sensor in the tire or on the valve stem measures the air pressure (and maybe also temperature) in the tire and transmits the data to the car over a wireless link. Since direct sensors are often hard to get at except for when the tire/tyre is off it makes sense to consider service at that time. The reason they need "service" is because they are battery powered. The devices tend to be built so maximize battery life and according this article from The Counterman lithium ion batteries are used with 5-6 years being typical.
Indirect (iTPMS) systems, use the ABS sensors on each wheel to infer tire pressure. This has the advantage of requiring no additional hardware and no ongoing maintenance, at the expense of being unable to measure absolute pressures and the possibility in some implementations of the system providing a false "ok" indication if all tires are losing pressure at the same rate.
The reason why you would need TPMS service is mostly likely because the battery in one or more of the in tire sensors of a dTPMS system was low or because a unit had failed. The shop should be able to show you this in advance of working on the car. With iTMPS there is no need for service at the time you get tires – at least not in the sense that you should do the work then so that you can save the cost of unmounting a tire to replace damaged sensor or one whose battery had died.
Usually when mounting tires you replace the valve stem, on in-wheel tire pressure monitors you replace the nut, schaeder valve, and gasket. Though this isn't true for all in-wheel monitors, on Fords the monitor is sometimes strapped to the wheel with a metal band and just requires replacing the valve stem. GM's usually use a replaceable rubber valve stem that's bolted to the monitor with a small torx screw.
Depending on what they quoted you per wheel, usually a few bucks for the kit, it's worth doing so that in the future you don't have to break the wheel/tire down again when they eventually leak.
Also depending on where you live if the nut that holds the monitor in place is too corroded and there's a risk of breaking the stem it's either left alone or the whole assembly is replaced.