Tablet batteries are typically two to three times larger than phone batteries, since they need to power much bigger screens. Filling a larger battery in a reasonable amount of time naturally requires more charging power.
While a phone might fast charge at 20-30W, many tablets request power in the 30-45W range; some large-screen professional tablets can demand as much as 45-65W, closer to laptop territory.
Charging a tablet with a low-power charger (say, 10W) will not cause harm, but it noticeably extends charging time; in fact, if the tablet is being actively used (watching video, gaming) some low-power chargers cannot keep up with consumption and the battery level can actually keep dropping.
Choosing a charger with the right power level for a PD-enabled tablet shortens charging time and keeps charging stable even during active use. Checking the maximum input power listed on the tablet's box or spec sheet is the right approach.
Charjett's 45W-and-above GaN chargers are built to meet this higher power need for tablets, while still working seamlessly with lower-power devices like phones thanks to PD negotiation.