it is extremely helpful to be able to link two list items (tasks) to each other without making one the dependency of the other. This is an increasingly popular feature in content, contact and task management in which a link from one item is visible in the other, and vice versa. Its called Two Way Reference.
For example, this is useful to relate two contacts with each other (I know John from Eve, or Ann is the mother of Jimmy, or this tasks relates to another) and the link can be made visible in the custom task column.
The functionality is as follows: When custom field Reference is added, there can be a autocomplete search box that allows to find any task (list item) anywhere, and when selecting one, it will make a link visible in both the current and the target item.
A more advanced way of implementing this is the creation of relationships, which means instead of just two way referende links, the relationship is a separate list item which connects two other list items and can hold the information of the relation such as a title, details etc.
As project manager and webdesigner the Reference as well as the Relation features are the most powerful and popular tools I implement for content and contact management