Relay is cheaper than an ECU
So, you know how your ignition coil works, yes?
You 'charge' up the coil then, when you 'open' the connection you get a huge high voltage discharge through the spark plug
Well, a relay coil does exactly the same thing - smaller coil of course, so the 'charge' potential and resultant discharge is less than your ignition coil - however easily still potentially couple of hundred volts.
You really do NOT want that energy discharging through your ECM
Trust me - if Triumph was confident in using a cheaper relay, they most certainly would!