Balancing ensures that all cell groups (for 36V you have 10 groups in serie A group could be 4 or 5 cells in parallel) remain equally charged
Without a balancer, if one group has a lower voltage, it will stay like that as they all receive the same amount of energy. The group with the lower voltage should get a bit more energy to come to the same charge state as the other groups.
In an ideal world, with a pack with all equal cells, you don't need a balancer as all cells will discharge equally and recharge equally. So the voltage of the groups should also stay equal.
BMS normally need 5V as supply voltage to function. Instead of making that voltage from the battery voltage, some just use the voltage of the 2 lower cell groups to make their 5V The voltage there is between 5V and 8.4V, so it's easier to use. This however also means that the 2 lower groups will become lower in voltage compared to the others as they have to power the bike and the BMS board. The BMS power consumption is low compared to the bike, but after a longer period, it will start to show a difference.
A good BMS is what protects your battery against most mis behavour. It controls maximum charge current and maximum discharge current. It als disconnects the battery output if the voltage becomes to low, and it disconnects the charger port if the cell group voltage has reached it's "charged state" If it does it's job properly, it will minimise the risk of your battery catching fire.
The problem is that some chinese BMS boards don't do their job properly. So buying a cheap BMS board from an unknown source is a risk. Some allow the cell group voltage to raise up to 4.3V. This will seriously reduce the number of charge cycles before your battery will start to degrade. The best BMS boards are those with a bluetooth interface. They allow to monitor all the battery parameters from a pc or a phone. So you can simply see the voltage of every cell group. You can also adjust the parameters like maximum discharge current and low voltage cutoff treshold. As usual, with great power comes great responsability. If you set the low voltage cutoff treshold of a 36V battery to 20V, you will damage your battery, but if you know what you are doing, the option to see all battery parameters is great.
And also very important, get a BMS that also controls your battery temperature. When something goes wrong, the cells will start to heat up. They hopefully don't go in thermal runaway right away if you stop the charging / discharging when a raise in temperature occurs.
So my advice, if you have room for one in your battery case, get a bluetooth BMS with balancing.
(I don't sell BMS's and I don't make them. It's just my opinion that you shouldn't go low cost on safety.)