What can weka do that python and sklearn can't?
Weka's decision trees are from the Quinlan family, whereas sklearn uses CART.
The most notable difference is that Quinlan trees aren't restricted to binary splits: a categorical column will be split into subtrees for each level.
Another is how missing values are dealt with, but there are some differences in individual implementations, so it's not straightforward to compare the two branches.
https://stackoverflow.com/q/9979461/10495893
TDIDT Decision Trees algorithm
Otherwise, I expect the main real difference is in whether it's easier to deal with python or java.  If you want to extract decision rules, you may be looking to post-process a decision tree; I know of skope-rules to do this in python, but not whether such a thing is easy in weka.