It's a very good way to make sure that if those clients need to route their internal networks to each other, they don't have to renumber. If two companies using 10.x.y.z networks merge, you are likely to end up with a mess. Heck, if you need to VPN to a 10.x.y.z network and your local network is also 10.x.y.z, you'll end up with a mess.

(The IPv6 solution here is Unique Local Addresses, where fdXX:XXXX:XXXX::/48 are all permissible local networks, and if you use a decent RNG to generate the 40-bit number XXXXXXXXXX, you're unlikely to hit a collision with any other actual, active site, let alone one you might want to route to.)