In Ohio and a few other states, a remnant of old English property law remains called dower. In essence, it means that your spouse has a potential interest in your property even if he or she isn't on title. If you are closing on a property in a state that recognizes dower, your spouse needs to come to closing, either to sign the mortgage (if you are buying with financing or a refinancing an existing loan) or the deed (if you are selling).
The story goes (I have no idea if this is true) that dower was a form of protection to keep an unruly husband from gambling away the homestead. Whether that's true or not, the point is that the spouse has a potential future interest in the property. If you're signing a mortgage, the spouse has to sign (called "releasing" dower) so that the mortgage interest is ahead of the dower interest. Otherwise, it will create some problems for the lender if the lender needs to foreclose. The same idea applies to deeds - the dower interest has to be transferred/released.
So does your spouse need to come to closing? Well, if you're in a state that recognizes dower such as Ohio or Kentucky and you're doing anything other than buying a house for cash, then yes.
That usually is only going to apply in situations where you're selling your residence. Why? Because you should have your investment property held in an LLC, trust, or similar entity. Trusts and LLCs don't have spouses, so there's no dower interest to release. So that's one time when your spouse doesn't need to come to closing even in a dower state (unless he or she is the trustee or the signing member of the LLC, of course).
Please remember that these posts are for informational purposes only and should not be considered or relied on as legal advice. If you are ever in doubt about your rights under the law, you should consult an attorney familiar with the law in your area.