Without the [ ] Excel thinks it's a 'normal' date/time ie 24 hrs in a day, so once 24 hrs rolls over it has to increment a day & restart the hours count. No different to a stopwatch: Once it goes past 9/10's of a second it increments by one second & the tenth's start again at zero. Ditto the seconds --> minutes etc.
The [ ] tells Excel not to increment the day once the number exceeds the increment value, but to continue to count using the same units.