More capacity is more things to break and fix.
If it's underutilized, you can save money by delaying replacement.
Let's keep things simple and say in one interconnect you'e got 4x 10G ports. If your utilization peaks at 5 Gbps, you way overbuilt that connection, but if one port breaks, you don't need to fix it until another port breaks; you can wait until three out of four ports are broken if you feel lucky. That delays hardware replacement and eliminates service trips and probably reduces technician time. If that connection peaks at 35 Gbps, you should fix any issues ASAP. (And start planning for capacity increases).
Of course, if the connection is to another network, they may bill for usage too; although I'm pretty sure US dominant ISPs are not paying for usage on most of their connections.