They are typically used when you have a lot of items in a list of some kind and you want to know if a particular one is present already without incurring the he

They are typically used when you have a lot of items in a list of some kind and you want to know if a particular one is present already without incurring the heavy lookup cost. When you check the Bloom filter it tells you: 1) it might be there or 2) it definitely isn't there. In the case of 2, you don't need to look it up. In case 1, you'll need to do the actual lookup. It is commonly used to filter high volume / frequency requests for something. For example, if you have a list of banned IP addresses, user accounts, etc, you can quickly go through the bloom filter without hitting the database.