moms that use dr. brown's bottles

ok ftm here, sorry if this sounds dumb 😂 so i got a pack of the 4 oz bottles and without reading anything about them i threw away the little insert that came between the nipple and the blue flow tube

well i got another pack of the 8 oz bottles and decided to actually read the thing that came with it and it said to keep it? i figured it was just extra packaging that they put on there but apparently its supposed to help it not leak? my question is, do they leak a lot if you don't use the inserts? i kept the ones that came w the 8 oz bottles so i do have some and i was just currious as to if i actually needed to keep them or not