Because nothing survives a bad design. Sweet deal, cheap price, best equipment ever — none of it matters if the system is sized wrong. You'll have a confusing, frustrating, and long-term costly experience anyway.
A proper design accounts for four things: (1) your current usage, (2) your utility's metering program, (3) when you use power, and (4) what you're adding in the next 18 months — EV, hot tub, heat pump, more kids. Under net billing, this requires your utility's Green Button data — 15-minute interval usage — fed into design software that models sun data against your actual life. If a rep sizes your system off one monthly bill total, they're guessing. Guessing is how people end up with surprise true-up bills.