Yes while the employer may not be able to legally force you to handle emails outside of official work hours and doing so counts as working...
Depending on your job, if you don't check and reply to some time sensitive emails outside of work hours, you might a) become a roadblock on someone else's project that's due soon b) be late to respond to "emergencies" and this can affect how people perceive your overall effectiveness and performance.
Of course, company culture can minimize last minute requests, unrealistic deadlines, etc. to minimize these "emergencies" that need quick replies but many don't.