Character Modification Slots let you equip Upgrades that can lower fall health/stamina damage, weapon damage, maximum health, etc. Besides upgrading your weapons? this is one of the necessities to keep up on to improve your odds of victory in a gunfight.
Char upgrade slots
You get the first one right away, and the rest by getting both factions up to ranks 10, 20 and then finally 30. So sometimes favoring one faction over another to get weapons isn't always the best route especially earlier on.

Some common character modifications you will see are:

  • Monolith (Increased Max Health)
  • Fast Regen (reduces health regen time)
  • Survivor (Reduces bullet damage)
  • Energizer (Reduces LTL stamina damage)

Some lesser modifications are:

  • Field Supplier (usable resupply crate to resupply or change weapons in battle)
  • Happy Landings (take less damage from falling)
  • Take a Knee (reduced stamina regen time)

Reasons are obvious, they all help you live longer. If you haven't unlocked the upgrade you want? the marketplace will have them.. but for a marked up price.. so it's time or money that are the deciding factors.