I would have to say I really do not understand why this move would make sense. Yes Weegar is very good, but so are Nylander and Sandin, so what is the benefit?

There is a few reasons. Leafs gain the desired cap flexibility without getting rid of someone like Kerfoot who is extremely versatile. They gain 2 local boys in this trade who are likely to sign reasonable contracts to play for their childhood teams. Calgary gets some scoring in Nylander, who also has ties to Calgary, they get a penalty kill d man, and a puck moving d man who may turn out to be something yet in Sandin.

I am doing trades like these as I am tired of the usual Miller and Chychrun stuff.

This weakens the team significantly. Weegar is good but he's not going to add as much to the team as Nylander does and Toffoli is a decent depth scorer but he's not going to make anyone around him better. Nylander certainly does. And Sandin has so much potential. Look at the list of defenceman who have won the top defenceman at the world Juniors and tell me Sandin shouldn't be kept.

I think Nylander + Holl vs Weegar + Toffoli is a good trade

I will never use the World Juniors to determine a defenseman's worth. Pretty tough to judge. Although like you say there are some success stories... there are also some busts. D men take a long time to develop at the NHL level.
Finally, I am sorry to say but I don't see a lot of evidence of Nylander making those around him better. He is very skilled. But he definitely needs a strong linemate to be with.

My issue with this trade is the leafs are giving up the best player with two years term and the second best player as a young RFA, vs the Flames best player just 1 year

Not some, all but one of the last 20 winners have become at the very least, extremely good defenceman

You are overvaluing Sandin, he is fine, not "very good" the same way Valimaki is fine and not "very good"