When children leave home to go to college, they generally return during holidays, semester breaks, and summer vacations. If their former room is not waiting for them - or at least a reasonable alternative for them to live in while they are temporarily present during breaks and summer vacation - it sends a signal that they are unwelcome.
If more than one child has left home to attend school or pursue other pursuits for an extended period of time, all of the rooms that they occupied in their parent's homes don't necessarily need to be preserved and maintained, but there does need to be some accommodations for them when they return from time-to-time.
It might be that one former bedroom can be turned into a sitting area, media room, sewing room, den, home office, gaming center, or hobby area while other bedrooms are maintained for any and all adult children when they visit.
It might be that those children who have left home will return at some future date with families of their own, and sleeping accommodations will need to be provided for them and any children (grandchildren) who might be present.
Even with children who leave home and don't return right away to visit - or move back in as many of them are doing after finishing college, not finding gainful employment, or suffering a failed romantic relationship - there is still that occasional visit that parents long for and welcome when it occurs.
There also are visits from relatives outside the home from distant cities. Aging parents, aunts, uncles, and siblings of the homeowners may decide to visit for a long weekend or to celebrate an important event in the life of the family. Rather than send them to a hotel, it's nice when they can stay with their host family. This enhances the visit (especially when it is short) and gives more visitation and conversation time.
Lastly, that seemingly vacated living space that was being occupied by the children when they were present full-time in the home can come in handy as a place for naps and convalescing from brief illnesses.
Never be too quick to reassign or absorb that "extra" bedroom space that results when children leave home because (1) they can return at any time and essentially any age for a brief or even more extended visit, (2) that space may be required for other overnight visitors and guests, and (3) it can provide additional sleeping quarters for either parent or other children remaining at home when needed.
As nice as the idea might seem for converting a child's bedroom into other space in the home to meet other needs or demands when that child moves away from the nest, it can't always be done seamlessly without considering the possible impacts and ramifications of doing so - especially with the real possibility that it will be needed for them again at some future date.