The "motivation", as you put it, is chiefly tone - pure and simple. Playing pickstyle gives you that crystal clear, ringing, cutting, "piano wire" tone that's difficult, if not impossible, to obtain playing with any other technique.
If you're the type of player who doesn't acknowledge pickstyle tone as legitimate and acceptable, then no amount of explanation and justification will ever convince you - in which case, this is a waste of time.
If, however, you really didn't know any of this, yet you have an open mind concerning it, then I would commend some of the best in pickstyle bass playing for your listening pleasure - so you can begin to get up to speed. While there are many pro players, even legendary players, who might fit the bill, I personally recommend listening to Chris Squire's playing during his heyday with Yes. So check out Fragile, Close To The Edge, and Relayer - among other classic Yes works - and ask yourself: 'Would it really be possible to generate such a crisp, crunchy, penetrating, propulsive, harmonically-rich tone, by playing only fingerstyle?'
MM