Why do the cavs do this?

By dismantling the Mobley-Allen front court, the Cavs increase their spacing moving Mobley at the five and allowing a proper wing to play the floor. There's an element of positioning here where Allen and Mobley occupy the same roles and thereby the same space on the floor. That gets alleviated here. This also helps Mobley as an offensive player as he can more effectively score on the inside or be a hub. Not too mention the potential spacing for Mitchell could be huge. Herb jones offers versatile point of attack defence that has been missing from this roster, and allows the Cavs to cover the elite guards of the east. The added draft capital give roster flexibility that was non existent prior. It's also worth noting that Allen is not a particularly rare archetype, especially compared to Herb. Therefore, it may not be hard to get a cheap replacement to run 2-big defensive lineups.

Why do the pels do this?

Jarret Allen is a perfect fit and his rim protection is hugely impactful on the defensive end. His ability to effectively score as roll man make the Zion-Allen two man game lethal. While herb is elite, the Pels have a lot of wings, and Dyson Daniels can take on that role as a switchable point of attack defender (all be it, to a worse extent). Additionally, the spacing gets better via positioning (a non-shooting, roll-man big is less of a space killer than a poor shooting wing). You can also fit more floor spacers in a lineup with Allen anchoring the defence as a rim protector.