Reading the PEP for adding it, it seems primarily concerned with mirroring how the C builtins generally don't allow it, for performance.

https://peps.python.org/pep-0570/#rationale

That being said, you can find quite a lot of uses in the modules written in Python - though I'm not going to pretend I can tell you the reasoning for all them.

  grep "/\s*[,)]" /usr/lib/python3.13/ --recursive --include "*.py" --exclude-dir site-packages

In my own code, the main use I've found is that they don't "reserve" a slot in kwargs - this is totally valid code:

  def func(x, /, **kwargs):
    print(x, kwargs)
  func(1, x=2)

You could've worked around it by taking a dict before, but it's just a lot friendlier being able to take args as normal.

> What's the advantage of stopping someone from naming the parameter when they pass it in?

There are cases where a parameter name is worthless on the calling side, this is especially common for unary or binary functions.

In that case, the ability to name the parameter on the call site just constrains the implementation, and the overrides (in the case of methods).

When parameter names are meaningless for the final result

  def abs_days_difference(date1, date2, /):
    # note that swapping passed params will yield the same result

    delta = abs((date2 - date1).days)
    return delta

  d1 = date(2023, 1, 1)
  d2 = date(2024, 1, 15)
  abs_days_difference(d1,d2)==abs_days_difference(d2,d1)

The function returns the absolute diff in the number of days, so date1, date2 are meaningless variable names. It looks like then introduce an order(could #1 be the earlier date, while #2 be the later one?). If you don't get users any options it is clear that the order is meaningless

Compare with

  def day_difference(earlier_date, later_date):

    delta = (later_date - earlier_date).days
    return delta

Note that now

  d1 = date(2023, 1, 1)
  d2 = date(2024, 1, 15)
  day_difference(d1,d2)!=day_difference(d2,d1)

If you function signature is day_difference(date1, date2,*) you have to specify params as kwargs only, removing the confusion:

  d1 = date(2023, 1, 1)
  d2 = date(2024, 1, 15)
  #this now doesn't work
  day_difference(d1,d2)
  # this works and is much clearer for caller
  day_difference(earlier_date=d1,later_date=d2)

Edit: this is wrong, I was thinking/merging the idea of the '*' into this one, my bad.

-- original below --

> What's the advantage of stopping someone from naming the parameter when they pass it in?

From what I have seen, this feature is mostly used for the opposite direction, to force the caller to use keyword arguments past a certain number of (or zero) positional arguments.