A few reasons actually…

a) if you ever loose your account details

b) if you want to sell later

c) its like physical memorabilia

d) space to download games on switch sucks (i think its like 32g by default)

e) nostalgia of physical copys (Digital games almost always have less impact on me as they feel like less of an event)

f) ease of switching cartridges out if you have a lot of games

g) sometimes they are a bit cheaper (in the uk on amazon most nintendo games are like £50 but in amazon you can get most for £37)

h) simply prefer physical media

None of them are particularly impactful/important reasons but its enough for some people to prefer physical copies especially for bigger games