Simple situation. I make a transaction and set to low fee by mistake. Now i need to wait days for processing. Why i cant just cancel it and resubmit with higher fee? Ok, maybe this could create problems for services that dont check conformations on deposits. Another option. Why i cant set higher fee for transaction that already in mempool? As i know there are special wallets that allow to do so. Why this feature avaliable in only several not popular wallets?
As for me, option to make fee higher for mempool transactions is profitable for everyone. Miners will earn more. Recipient will recieve money faster. Sender will be happy. Why not?
There is no way a transaction which is sent but not confirmed yet could be cancelled. If this was the case, then people would exploit this for double spending. If you don't want the transaction to get delayed, its best to use the default fee and do not change the fee to too low that it takes many days to confirm.