Staying on top of chain wear is crucial if you want to prolong the life of your drivetrain.
While there are other great tools such as the Park Tool CC-2 chain checker, Shimano techs will tell you (and their argument is convincing), that Shimano chains can only be reliably checked by the TL-CN42. This may sound like Shimano pitching their own tool, but they argue that any other tool has a tendency to recommend junking a chain prematurely. Considering the cost of a tool vs. a new chain, combined with Shimano' expertise, we agree it' worth it.
Checking chain wear should be part of your regular maintenance routine. Once you hit the wear limits, you reallyshouldreplace it with a new one. If you don't, you'll start putting extra wear on your cassette and other components, plus risk breaking a chain when it' least convenient.