Why distinguish legal freedom from the practical capacity to exercise freedom?

Because having a legal right and having the resources necessary to exercise that right are not identical things.

Example

Suppose the law says:

“You are free to choose your employer.”

Person A:

owns land;

has savings;

can farm independently;

can move to another employer.

Person B:

owns no land;

has no savings;

has no independent livelihood;

has very limited employment opportunities.

Both have the same legal freedom.

But Person A has a much greater practical capacity to exercise that freedom.

That is the distinction you are making.

“Analytically distinguishable”

This simply means:

For purposes of analysis, we can separate the two concepts even though they are related.

So:

Legal freedom is what the law permits or guarantees.

Practical capacity is what a person is actually able to do given their resources and circumstances.

They are related, but they are not identical.