The reduction in transistor size means that newer GaN USB chargers can be physically smaller than the older silicon technology chargers. And while it's nice to have smaller chargers, it is the increased efficiency that is the most important factor when it comes to USB chargers because the more efficient an electronic component is, the less waste heat it generates.
And the less waste heat generated, the lower the chances of overheating and the less cooling that is required to keep the charger operating safely.
Consumers are, understandably, concerned when chargers feel hot to the touch. While it is common for chargers that use silicon transistors to get to the point of almost being too hot to touch, I find it rare for a charger using GaN technology to feel mildly warm.
The faster switching also means that a GaN transistor inside a charger can have better control over the charging and respond to events such as overheating or overvoltage much quicker than older transistors could.
This greater efficiency and faster switching are critical for modern USB-C chargers because USB-C carries even increasing power loads, with 100W loads per port and above now being commonplace, and 240W chargers soon to be a reality.
Another advantage is that a single charger can have multiple high-output ports, such as the Ugreen 300W 5-port GaN desktop charging station, which has USB-C ports capable of 140W and 100W, so one GaN charger can replace a whole pile of older chargers.