When running an active node (BFT leader or stake pool) it is interesting to be able to make choices on how to manage the pending transactions: how long to keep them, how to prioritize them etc.
mempool field in your node config file is not mandatory, by default it is set
mempool: pool_max_entries: 10000 log_max_entries: 100000
pool_max_entries: (optional, default is 10000). Set a maximum size of the mempool
log_max_entries: (optional, default is 100000). Set a maximum size of fragment logs