There are two main ways to subdivide content – subdomains or subdirectories. I will henceforth refer to “subdirectories” as “folders” to avoid confusion between two similar “sub” terms. Folders are easily understood. They appear as slashed organizational groupings on your domain, just like folders on your PC. If you have several events to share, you might group them all under an events folder.
Content organized by folders
Is there ever a time when you wouldn’t want to use folders, but instead a subdomain?
Content organized by subdomains
The short answer is: No. I never advocate for the use of subdomains. Subdomains divide your domain in the eyes of Google. You must monitor and maintain the authority for each subdomain separately. Your ranking can suffer as a result. Most small non-profits already struggle to gain authority with limited content and backlinks. Dividing it even further makes no sense.
There are times, however, that require a subdomain. Subdomains are configured easily across different platforms or technologies. That is typically the #1 reason they are used. It usually stems from someone wanting to create a new or different web experience for a particular program or event. They might go out and purchase a dedicated (different) domain. Instead of having a different domain, subdomains can bridge the gap between them while maintain your branding and identity. If WordPress hosts your site, but Squarespace hosts a new event page, placing them side by side on your domain only requires a single DNS setting.
The best thing you can do is anticipate these needs, get in front of rogue web activities, and concentrate all of your content and users onto a single platform and domain. Your audience easily finding your content more than makes up for the small sacrifice in autonomy!