The benefits of keeping it aside to feed the plants:
‘the problem with urine is that it is the main source of some of the chemical nutrients that have to be removed in sewage treatment plants if they are not to wreck ecosystems downstream. Despite making up only 1 per cent of the volume of waste water, urine contributes about 80 per cent of the nitrogen and 45 per cent of all the phosphate. Peeing into the pan immediately dilutes these chemicals with vast quantities of water, making the removal process unnecessarily inefficient.’
from the New Scientist on urine-separation toilet or NoMix toilets