Why is the magnetic field around a bar magnet represented by iron filings?
There's a general problem in physics that you can't measure anything without changing it.  Any attempt to introduce a measuring instrument into an experiment changes the experimental setup, and so alters the results.

So the aim is to choose a measuring instrument that changes the results as little as possible.  For example, a voltmeter is designed to have as high a resistance as possible, so it doesn't add any significant load to the circuit it's measuring.

In the case of iron filings, they are small and either not magnetised, or are only weakly magnetised.  The bar magnet, on the other hand, is much bigger and strongly magnetised.  So we assume that the iron filings have only a small effect on the field that they are measuring.

You can make any effect smaller, by limiting how many filings you use.  Just sprinkle a few around, and they will have little effect.  Dump a whole jar full of them on top of the magnet, and you probably have completely changed the field you were measuring.