Okay the TIE Punisher, at 21 points it is the same cost as the shuttle and the TIE Advanced. However it has 1 more hull and 2 less shields from the shuttle as well as 1 less firepower. It is in that funny spot at 21 points between shuttle and advanced.
So as the Shuttle well it can be a durability tank with 10 hit points, it has considerable firepower at 3 with a cannon also with sensor and crew slots it can bring some considerable firepower to the table. Disadvantages would be a restrictive dial requiring the EU upgrade. The large base makes it easier to target but also with EU gives it more maneuverability.
As for the TIE advanced it gets a sensor upgrade for free and has the barrel roll ability so giving it EU makes it practically a more durable arc dodger unlike the glass cannons of the TIE Interceptors. More of an agility tank.
As for the TIE Punisher it cost more than an advance, while it has the boost action so it save a point on EU but doesn't save anything on sensor upgrades. It has 1 agility so which makes it easy to focus down, autothrusters can make for a better chance that it could evade 1 of the shots. IT lacks the firepower of a shuttle and its boost will be behind the shuttles range of maneuvers. No cannon upgrade means for firepower it will have to supplement its 2 firepower with sub-optimal torpedo and missile weapons and bombs to hope and get a leg up on damage efficiency.
or
because just ****... that glory .