In many cases when the trial data are collected from an online log (or a sequence of events coming in real time) and metrics are calculated over the periods of time of a different duration (like Retention D1 and Conversion D2 with 2 days for the first and 3 days for the second necessary for calculation) it is advantageous to collect data related to the same daily cohorts only. This can be done by starting a trial for a day, then pausing it until the metric with the longest duration can be calculated, then, after the intermediate evaluation - restarting the same trial.
It can also be used for collecting small samples from a large stream of data periodically, let's say once a day for 5 minutes, then assessing the resulting data together, as a single Trial.

Subclassing works of course, but it is just logical to have it in the framework itself.