There are a couple reasons:
Reducing account creation cost by requiring a delegation of SP alongside account creation. This allows us to make millions of accounts without spending millions of dollars, which is super important.
SP delegation allows @steemit (which currently holds a huge amount of the platform's stake) to delegate SP to a broad swath of the userbase (as a part of an achievements system, for example, driving retention and re-engagement), reducing the relative influence of a few dozen large holders of SP by creating a sort of SP "middle class" (via delegation to active, engaged users).
For a quick off-the-cuff example (all details subject to change, this is just a quick hypothetical), imagine an achievement called "On That Grind" that is given to a user who makes one original post per week that yields >$10 in rewards, for four weeks straight. A set of such users would be a great group to bestow stake delegations on, as they are contributing meaningfully to the platform and everyone would benefit from giving them increased curation influence.