3 reasons that I can think of: (1) For mostly static screens that the GPU's frame buffer (and pipeline) could be empty and there would be nothing to process.

3 reasons that I can think of: (1) For mostly static screens that the GPU's frame buffer (and pipeline) could be empty and there would be nothing to process. (2) The font rendering algorithm will have to run every second. These are not simple bitmap-type monospace fonts, and thus the calculation of the output will have to be done every second and that these rendered digits-combinations likely aren't cached. (3) The thread for system clock displaying system can set a sleep for 1 minute (after it has aligned to the start of the minute (or in actuality the cron-type service underlying operating systems is likely used here for which the alignment to the minute start is already a given)), instead of 1 second.