Why does Tor save the circuit build times and what does it want to learn out of them? Tor loads the circuit build times from a file at the beginning. I do not know why it needs them.
The Circuit-Build-Times are measured and saved for the CircuitBuild-Timeouts. So Tor is logging the time it takes to build a Circuit. The CircuitBuild-Timeouts are then used to eliminate circuits which need excessive time to build.
The assumption is, that circuits, which need such a long time to be build, are more likely to be highly congested. So the purpose of not using slow-building-circuits, is get the better performance for the user as well as to relieve relays which are used to capacity.
CircuitBuild-Timeouts differ for every user because they depend on numerous factors (such as the local connection to the internet, the state of the tor-network, the state of the relays in the circuit and much more). This is why the 1000 most recent circuit build times are gathered and stored.
For more information read the Path-Spec