Why would i need to replace rear crank shaft seal and repair transmission output on 1997 Chevrolet Astro

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shaft when there's no obvious oil dripping on garage floor?

Are these repairs necessary for an old car?

Asked by for the 1997 Chevrolet Astro
If the transmission is being repaired and is already out of the vehicle it would be prudent to replace the rear crankshaft seal as if it leaked at a later date the whole transmission would have to be removed to replace it. The cost of replacing it now with everything removed is minor by comparison.
To clarify, these are suggestions made by a mechanic during an oil change and not because i needed any transmission repair. I don't know how he came to the conclusion that such repairs/services are needed as I did not notice any leaking of oil on the garage floor or anything wrong with the car. I was told that it would cost about $2000 for the repairs. My question is: do I really need to have those repairs done?