Why would you want to know if it causes you no problems and there is currently no cure.
Unless you want to vegetate on the sick for the rest of your life.
Because it can prove very useful for to know you've had something if you get ill later on, or if they find out it can lead to an issue the doctors can keep an eye on it.
There are a bunch of virus's that are known to make it far more likely you'll get other problems later on, or more susceptible stuff.
For example we now know that a virus is basically responsible for cervical cancer (yay vaccine to help prevent (a) cancer!), we know that another one can cause long term issues with your immune system, and a bunch of others can hang around and cause secondary seemingly unrelated problems decades later (measles/shingles IIRC).
This doesn't take into account the virus's that we now know cause damage to organs that mean you have to be more careful with what you eat and what medications you take in the future such as the various ones that attack the liver as a primary target.
Given covid does damage organs albeit not always enough to have an obvious and immediate effect, knowing you've had it gives your GP's and yourself a head start on monitoring for anything that is found to be linked to it in the future, so if you get an odd result during a routine blood test it might shorten the diagnostic processes.
Basically, the more you know of your medical history the more you and your doctors can do to hopefully avoid future problems, or keep an eye open for the early warning signs.
It's pretty much the same reason medics ask about all sorts of personal and family history, knowing you've had X in the past (or your close relatives have) can be a very good indicator that you're at higher risk of it or something related to it.
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You don't need to worry about your test result showing you've had something, but it's far better to it for future diagnostic purposes than not.