For small offices, a full PC may be overkill if they don't have an administrator or dedicated company with short response time to administer it.
Dedicated units, once configured, are lower power, no moving parts, are reset by power cycling and generally "just work" at that point.
PC's are more flexible, but have moving parts (and more points of failure) and have lots of features that either aren't fully used or needed or are confusing for them to use (without an administrator in-house). It uses more power. It can be noisier. It may get re-appropriated by staff thinking it's just a computer that's unused, or powered off, unless someone is there to slap hands away or make sure a big note is taped to the front. It takes more space.
In the long run unless there's staff for maintaining it the added cost of a dedicated unit is generally a peace of mind tax. You pay extra to keep from having the small office calling and yelling about unknown failures or difficulties in using it.
Also the dedicated units usually have warranties and service support. Do it yourself routers, not so much.