61.57(a)(2) only applies to regaining currency if you are flying a plane that requires more than one pilot crewmember. It allows a non-current pilot to act as PIC when it would otherwise be illegal in order to make it easier for a non-current pilot to regain currency. It has no relevance to a solo pilot in a plane certificated for a single pilot, since 61.57(a)(1) does not forbid a non-current pilot from acting as PIC in a plane certificated for a single pilot as long as they are not carrying passengers.
61.57(a)(1) forbids you from acting as PIC if you are carrying passengers or if the plane requires a multi-member crew:
"Except as provided in paragraph (e) of this section, no person may act as a pilot in command of an aircraft carrying passengers or of an aircraft certificated for more than one pilot flight crewmember unless that person has made at least three takeoffs and three landings within the preceding 90 days, and—"
61.57(a)(2) allows you to operate during the day with additional required crewmembers, as long as you aren't carrying passengers or property. This makes it easier to regain currency once you have lost it if you are flying a plane that requires multiple pilot crewmembers.
61.57(b)(1) is different in that it only forbids you from acting as PIC at night if you are carrying passengers. It contains the same language as 61.57(a)(1): "[...]no person may act as pilot in command of an aircraft carrying passengers[...]" (emphasis mine), but it does not have the bit about aircraft certificated for more than one pilot crewmember. Since it doesn't have this restriction, it doesn't need a similar exception.