When partner jumps to 2NT after our 1-minor opening we don’t have a lot of space to communicate below 3NT. It is important to have good agreements about the follow-up bidding.
When partner opens in 3rd or 4th seat and we have a good hand, we do not want to go jumping around to show it – partner may have no game interest for their opening bid. We want to have an effective way of describing our good hands cheaply when partner opens in 3rd or 4th seat. When partner opens 1-Major and we have a maximum passed hand with a fit, then we want to be able to show this to partner without getting too high. This is what the Drury convention accomplishes.
One of the most important parts of bridge is raising partner. We try to raise Opener whenever possible. We may choose not to support partner immediately if we feel we have something more pressing to communicate, in which case we can respond in our own Major, use a negative double, or bid some number of notrump, but in general we strive to “support with support.”