Something like spaceflight is subject to chaotic forces and unpredictable interactions, so having that many digits (more than 15 decimal) becomes negligible. (For example, a planetary force will probably vary by that much in ways models don't capture, like subtle tidal forces and subtle variations in its orbit, etc.). Navigation usually involves methods of measuring your position and adjusting course in theory needing much less precision.
Simulating physical systems to extremely high precision (e.g. more than double precision) in general seems pointless in most situations because of those effects.
Adding on to this - the physical characteristics of the spacecraft itself are also not machined to nearly 15 digits of tolerance, so these feedback systems are necessary even if the rest of the universe were perfectly modeled.
Simulating physical systems to extremely high precision (e.g. more than double precision) in general seems pointless in most situations because of those effects.