I am not using it to store funds. I only came across it when I was reading and was curious to know where the transaction fee goes to and it was written there as optional. What if one doesn’t include any transaction fee since it is optional, but thanks to @Hyphen(-), I know why it is written as optional now.
More info: https://en.bitcoin.it/wiki/Miner_fees#Priority_transactions
But now, although still valid, it's "non-standard" so mostly all nodes wont relay a tx without fee and miners will most likely reject it.
The target is just a number stored in the each block header and as you know it,
a miner who'll generate a hash of their candidate block's block header that's lower than the target will be able to include their block to the blockchain.
Target adjusts in every 2016 blocks which is roughly 2 weeks, the adjustment is based from how fast or slow the previous 2016 blocks were mined.
More info: learnmeabitcoin.com - How is the target calculated?
(you can use the search bar of the link above if you want to know more about Bitcoin)