What traditional demand is
Traditional (non-programmatic) ads are used for a few purposes:
Filling "unfilled impressions" - the white space left where no programmatic buyer purchased the impression.
Running your own direct, non-programmatic advertisers with image creatives at a fixed CPM.
How to decide
Sites differ in content and advertiser mix. If a direct advertiser pays a high enough CPM that beats your average programmatic CPMs, it can be worth selling those impressions directly instead of through a programmatic buyer.
Passbacks
A passback is what happens when an impression is not filled by one demand source and is passed to the next. Traditional demand and passbacks work together: when a programmatic buyer does not bid, the impression can pass back to your own traditional advertiser or house creative rather than going unfilled. This helps eliminate white space and make more of each pageview count.
Note: you cannot insert scripts or programmatic ads directly into the auction. Sulvo's programmatic connectors (Prebid and special connectors) are quality-tested, maintained and hosted by Sulvo to protect your ad slots from malicious or broken third-party code.
Related
See "What are Prebid connectors?" and "What types of ad formats can I use?".