It provides parity to the Self-Employed, to match what employer's are allowed:

"The deferred deposits of the employer's share of Social Security tax must be deposited by the following dates (referred to as the "applicable dates") to be treated as timely (and avoid a failure to deposit penalty):

• On December 31, 2021, 50 percent of the eligible deferred amount; and
• On December 31, 2022, the remaining amount."

From: https://www.irs.gov/newsroom/deferral-of-employment-tax-deposits-and-payments-through-december-31-20...