Since this is an evaluation board there are a bit more components than strictly necessary. C253, C254, C255 forms a pi-filter which can be used for filtering and/or matching. R256 and C257 will typically be used for antenna matching. R256 doubles as a "switch" between the PCB antenna and the SMA connector, depending on which direction it is soldered. Only a pi-filter between the balun and the antenna will in most cases be enough.
The SMA-connector is for testing. It is easier to connect the board to spectrum analyzers etc. when you have a connector.
It is not a requirement to put A 50 ohm trace in the design. Rather for RF traces of a certain length it is a requirement that the trace impedance is50 ohm to avoid reflections etc.
/Fredrik