A new targeting option enables Heli to seek and pursue the Bloons automatically.
Pursuit is the second upgrade on the first path for the Heli Pilot in both BTD5 Mobile and BTD6. It gives the Heli Pilot a new target priority: Pursuit. Pursuit target priority allows the Heli Pilot to follow bloons that are the closest to the exit, equivalent to the target "first". This is automatic, no touch is needed. So it can be really useful if you don't want to be busy controlling it.
In BTD5 Mobile and BMC, the current setting of the Heli Pilot would need to be changed to activate Pursuit mode; in BTD Battles Mobile, it automatically changes to Pursuit Mode upon purchase of the upgrade. In the BTD5 generation, Pursuit would typically be around 50 pixels away from the most first bloon on at any angle from the point of the targeted bloon, which typically rendered Razor Rotors less reliable on this setting. In BTD6, the Pursuit targeting has improved so that it will follow along the track as close as possible to the front of the bloon, clunching with other Heli Pilots on Pursuit in the order of whichever Heli Pilot went to Pursuit first.
This was formerly the only upgrade that gives a tower a new targeting option. When the Monkey Sub was added to the mobile tower roster of BTD5, it too got a targeting option granted to it by an upgrade.