In the past you hardly ever needed the number.  Some race organisers use the runbritain online entry system which is available to any race that obtains its UKA Race Licence through runbritain.
The online entry system asks for a runner’s unique runners number (URN) which is then automatically checked through the England Athletics database to ensure that a runner is genuinely affiliated to the governing body. This is important from a race organiser’s perspective because it ensures that the price difference between affiliated and unaffiliated runners (usually £2 as of 2019) goes to the race.  Organisers are increasingly likely to use this system to check runner’s affiliation status.