The first thing to settle is that Higher Level is not worth more points. A 7 is a 7 on both levels, and the 42 available from subjects does not distinguish between them.
They are equal in points
Since the point value is identical, a student who takes an easier route to a 7 at Standard Level scores the same as one who fought for a 7 at Higher Level.
If points were the only consideration, the strategy would be obvious. They are not, because of what comes next.
The first difference: content load
Higher Level subjects carry roughly twice the teaching hours and go considerably deeper. The assessment includes an extra paper in most subjects.
This means a Higher Level 6 usually represents more work than a Standard Level 7, and it also means three Higher Level subjects chosen without regard to workload can pull down the other three.
The second difference: university requirements
This is where the choice actually bites. Universities write offers that name subjects at Higher Level: engineering programmes commonly require Higher Level mathematics, medicine requires Higher Level chemistry and often biology.
A student who took the subject at Standard Level does not fall short by a grade; the application is simply not eligible. This is a closed door rather than a lower score.
Changing the choice later
A move from Higher Level to Standard Level is usually possible early in the first year and becomes difficult after that, because the syllabuses diverge. Moving the other way is harder still.
Schools set their own deadlines for changes, and those deadlines are earlier than most students expect.
The order the decision is made in
Intended field first, then the Higher Level subjects that field requires, then the remaining three. Making the choice from which subjects are enjoyable produces a combination that can close a route two years later, when nothing can be done about it.