Tough question. There is no consensus about time tracking in agile methodologies. Some people think agile methodologies are not meant to include time tracking, while some really need time tracking for a variety of different reasons. Either way, Taiga is built to accommodate multiple use cases.
But, of course, agile methodologies are meant to be malleable, so even if we decided to not implement time tracking as a feature in Taiga, teams are implementing it by using workarounds (like adding hours in the task or user story title, etc.)
To accommodate these practices we came out with something that could fit in any team workflow and solve many different problems: custom fields.