In older versions of Soulseek, you had to have two consecutive port numbers (eg. 20000 and 20001, or 12345 and 12346, etc). AirVPN was the only VPN that let you pick the port numbers, so you could guarantee getting two consecutive ports.
I don't think modern Soulseek clients require that any more (slskd only requires one port as far as I can tell) so it's not important any more, but AirVPN is still the most recommended by Soulseek users.
Private Internet Access only give you a single forwarded port, so you couldn't use it for both torrents and Soulseek at the same time (for example). AirVPN used to provide 20, but I think they reduced it to 5 for new accounts.
AirVPN's forwarded ports are also not server-specific; you get your chosen ports regardless of which server you use.