Relatively cheap ($5.4 atm) – compared not just to other Xiao boards but also to other controllers in general.
The form factor is about 1u (21 x 17.5mm) so it's really easy to fit into pretty much any layout. This parameter makes it a good alternative to other small controllers like the Tiny2040, the RP2040-Zero or the RP2040 Stamp.
The number of GPIO pins (11) may scare you off at first sight, but it's more than enough for splits (in theory up to 30 keys per half) even with a basic matrix.
However, and I just learned this recently, with a "real" duplex matrix (bidirectional scan) you can control up to 60 keys with this tiny board. This makes the Seeeduino Xiao RP2040 suitable for driving a 60%-ish keyboard and huge splits alike.
(To be able to benefit from the duplex matrix, at least at the moment, you have to rewrite your custom matrix scanning algorithm for most firmware I guess, except if you use PRK, where this feature is already implemented.)