Joint tenancy is most common among married couples because it helps property owners avoid probate. Without joint tenancy, a spouse would have to wait for their partner's Last Will to go through a legal review process—which can take months or even years.
A spouse cannot sell a home their spouse is living in (the homestead). Selling a homestead will require a court order from an Illinois divorce court. If both spouses are not on the deed and the property is not a homestead, the owner spouse can do whatever they want with the property in their name.
For instance, if you're married, the most common way to title your home is Tenancy by the Entirety (TBE). That endows survivorship rights, some creditor protection, and allows for transfers only with the consent of both spouses.
Tenancy by the entirety refers to a form of shared property ownership that is usually reserved only for married couples. A tenancy by the entirety permits spouses to jointly own property as a single legal entity. This means that each spouse has an equal and undivided interest in the property.
Joint tenants need not be married or otherwise related, and any sort of property, real or personal, may be held in joint tenancy. Only married couples may hold property as tenants by the entirety in Illinois, and they may use that form of ownership only for homestead property.
In order to sell or convey the entire property, all owners must join in the transfer. As tenants in common, the co-owners have the right to sell, gift, or transfer their interest in the property without the other owners' permission.
Finally, in my state of Illinois, unmarried couples can hold title as Tenants in Common or Joint Tenants BUT once they get married, they qualify to take title as Tenants by the Entirety, which they would do by signing and recording a new deed.