A lot of businesses tend to ignore the maintenance on their website and look at it like it's a one and done project. The expectation that a website will work without maintenance is unrealistic. Maintenance doesn't always have a tangible benefit that you can see. The benefits however include not dealing with downtime, bugs, or the site not working like it did when it first launched.

Technology doesn't last forever, this is common in all industries. Support for older operating systems on computers or phones is no longer available. CMSs also get retired, for Example ExpressionEngine 3 retired in December 2018 and Craft CMS 2 will retire in March 31 2019.

Older un-maintained sites may reach a point where it's not possible to upgrade. The only options are to rebuild completely or do band-aid fixes to keep the site limping along temporarily. By being proactive and maintaining a site regularly these sorts of things should not happen.

Continual development is adding in new features or options that were not available at the time the site launched.