Lithium can be damaged and ruined if discharged too much. They also do not sag like sla's. So they hold their voltage up until they are close to discharged. And when their voltage drops, it drops very quickly. Too quickly for a person to notice and intervene, so it needs electronics to protect them.

Its something like this, for lifepo4. For each cell, fully charged is 3.65-3.7v, any more charging and the voltage will just spike and ruin the cell. Off the charger they will settle back to around 3.3v, they might sag some on high current discharges due to internal resistance. When they are discharged the voltage will start dropping quickly. Bms systems will shut them off at 2-2.5v, any further discharge will ruin the cell.

On sla batteries the cells average out as you charge the pack, on lithium they will not. Each cell is in a different state of charge and will get full at different rates and can over charge while the other cells are still charging. This is why you need a bms for lithium. It monitor each cell for voltage and bleed off charge when full and shut off the system when empty.

I'm no expert, but this is what I've learned over the last year on these forums. So if I have anything wrong, the experts can correct me.