I am still in two minds as it happens but my reason is to make use of the upstroke. I feel it does take a lot out of my quads just using flats and presume its makes sense to use the hamstrings to share the workload. Plus the more I cycle the greater the strength imbalance between quads and hammies become. I dont think the leg angle is due to the knee, pretty sure its at the abductor/adductor area due to an old footy injury.
People say 'make use of the upstroke' but that doesn't pass any examination.
The pedals are linked, when you push down on one the other is being pushed up under power, so you cannot add any power to that by pulling your leg upwards.
The other point is that you have very little upwards power in your leg.
If I rested my palm on your instep while you were standing, you could not lift that leg.
But if I rested my palm under your sole, you could easily stamp down to push my hand out of the way.
The stamping down motion is where you have the power, which is equivalent to pushing down on the pedals.
I believe Sky - as they would - did some scientific examination of this and found a tiny, barely measurable benefit to being clipped in.
In their case, getting Bernal a metre further down the road over a Grand Tour stage could be the difference between winning and losing - marginal gains and all that.
That simply doesn't apply to the likes of you and me, but just to reiterate, personal preference rules.
If you try clipless and like the feel of them, use them.