Thats not buying land or real estate. They're called securities. As in Mortgage Backed Securities. LOL
As usual, banks will never tell you what they are actually doing, but you can get the concept step by step
From a higher level of abstraction, MBS is some kind of asset that has value, it does not matter it is gold or land or debt, they are all something with value. Issuing money backed by valuable asset is the spirit of Real Bill's Doctrine, MBS is the Real Bill here
From the name, MBS is a kind of security: A tradable financial asset, similar to bond. Bond is debt, backed by the promise of repay in future. So MBS is also backed by the promise of repay in future, but with one exception: It has a mortgaged house behind it. If the borrower defaults, bond becomes worthless, but when that mortgage defaults, MBS owner would take over the property as a compensation
So MBS is not only debt, but also a debt backed by the land. During a financial crisis this debt have a high rate of default (foreclosure), as a result the property will belong to MBS owner
FED purchased huge amount of MBS, because when house price crashed, many of these securities get defaulted and leave the banks with house. If banks sell these house on open market, they will crash the house price further and trigger a total melt-down of everything
So FED was actually buying these houses to prevent a house sell-off, but FED greatly benefited from this move and now all those houses are listed on the asset side of FED's balance sheet