What does SMA Tee Adapter do?

It just provides straight connections. If you know anything about RF then you should realize that it will only work properly for low frequencies. At high frequencies the characteristic impedance is affected resulting in reflections and unwanted attenuation of your signal.

I would only use these to split or combine a signal up to 100 MHz or so and even that is pushing it.

One good use of coaxial tee adapter is attachment of coaxial cable tuning stub or surge protection.