A lot of time-series data has complex relations and cross-correlations that make it unsuitable for column stores. Take for example a device that reports temperature and humidity. You may want to enforce foreign key relations between devices and readings, or you may want to do analysis for device in a particular location (which may be part of readings or devices table) secondary indexes help here. Or you may want to easy access to all data where temperature is above 90 degrees (partial indexes help here). You often want this in a real-time dashboard where the kind of latencies you see with MPP are problematic. Pretty much the columnar/MPP stuff is good for simple aggregates for ad-hoc non-real-time queries but TimescaleDB is geared to more complex analysis to power dashboards.

Plus, when you know which particular aggregates you want to regularly query, TimescaleDB makes it easily to perform continuous roll-ups so you can store in a separate (hyper)table.