I think oval handle reduces twisting which also imparts more power for off Center shots. This also mean that you don’t need monster edges. Also a thicker oval handle reduces tight bottom hand grip which in theory makes you avoid swish across the line too often (remember Adam Gilchrist - he used extreme oval handle and squash ball in his bottom hand). Other than that, it is always down to preference and these aren’t hard and fast rules. Virat Kohli used to use round handles but he switched to tapered oval handles around 2014-2015 and he is still one of the most wristy players going around. On the negative side, since you are using more of first two fingers on your bottom hand with the oval handle, a long innings always leaves these two fingers and the webbing between them of my bottom hand a bit sore and tired. I guess it can always be improved by practicing with grip exercisers.
Tendulkar never used an oval handle and his strokes were very well defined and straight whereas Mohammad Azaruddin used to get bats made by JM (extreme oval) and he was as wristy as it gets. In nutshell, use what makes you score big and feels natural.
Personally, I abhor oval and semi oval handles.
My problem is with the explanation behind the oval handle, which seems more of an oxymoron, where we are told that a semi oval or an oval handle reduces the slip and twisting and turning of the handle in hands. And hence when you play a straight drive, you get a more firmer grip and more power behind your shot.
Here is my logic,
First, if the bat slips in your hands? well, excuse my French but you are wuss. Don't play cricket with such weak muscles and such low energy levels where you can't hold the bat with a firm enough grip.
Second. you DO NEED to open the bat's face to play shot like a square cut, for example, and the round handle helps maneuver the bat A LOT easier than an oval, semi oval or a square or a pentagon or a hexagon shape. You see what I mean?