There are alternative 401K plans where you can hold things like real estate, gold, etc. in your 401K account ... though if it's an employer plan, I doubt they'd allow those options. Mostly people use these for tax shelters since traditional 401Ks are tax-free up front. If you put your house in your 401K, for instance, you can also effectively put money in there that you spend on maintaining it, remodeling, repairs, etc. That money then would be disbursed to the contractors, supply companies, etc. from within the 401K account and therefore escape all taxes. It's a secret way to take advantage of 401K regulations to turn them into tax shelters.
I used to work as a software developer for this company.
https://invest.trustetc.com/gsre-sdiraThere's very little oversight of the accounts, so people are basically using these account to launder money through them.