The RC time constant is saying how much time a 63% change takes.
Often the C value is known. Sometimes R is known, and other times it cannot be measured directly.
R might be a combination of (a) visible components, and (b) unseen internal impedance in inputs/ outputs of neighboring devices.
R might be one value while a device is 'on', and it might be another value when that device is 'off'. The two might alternate through a cycle.
We might want to know the RC time constant because we're interested in a particular frequency range, or a sample-and-hold time, or a propagation delay, etc.