The short answer
The oil is the preservative. A layer of oil over the pieces keeps air and moisture out, which stops mould and lets the achaar mature safely for months. It also carries the spice flavour, so do not drain it: use it in cooking instead.
What the oil does
- Seals the pieces from air and moisture.
- Carries the flavour of the spices.
- Keeps the texture of the pieces from drying out.
Can I drain it?
Only what you serve. Keep enough in the jar to cover the pieces completely, and use the extra in tadka, marinades or parathas.