In short, we need this number to compare how the pool changed over time. By the moment of withdrawing, we'll compare the pool's factor to the user's factor (got on a snapshot at the moment of entry into the pool) and estimate if that resulted in a gain or loss. The user can experience an impermanent loss or gain depending on the moment they entered and what happened to the pool while they were there. It is important to highlight that even if a pool has an impermanent loss, a user that just joined will not face immediate impermanent loss. Instead, each user will have its own path of impermanent loss or gain calculation in a pool.