1. To compete. With a good hand and/or a long suit the overcaller wants to compete in the auction.
2. To sacrifice. With a weak hand and/or a good long suit, a preemptive overcall can lead to a worthwhile sacrifice.
3. To disrupt. Where the opposition are looking for a contract, an overcall especially in Spades takes up valuable bidding space.
4. To indicate lead. With a strong suit but a weak hand, an overcall can tell partner which suit to lead in defence.
. . . five cards and at least two honours in the bid suit. With less than 10 points, six cards are required. Bidding with less is risky because you might be in a contract with an unsupporting partner. It is also unhelpful to partner if you do not have a good quality suit and she bids to game in NT.
The opener's partner has passed so bidding your best suit could find a fit and makes the opener bid agin, thus avoiding a cheap contract. This is called a protective bid.
If an overcall finds a fit, it can be a winning strategy in duplicate bridge, to bid the contract to one more than is likely to be achieved. This prevents opposition making part game.
. . . the overcaller has an opening hand but no 5 card suit. This bid promises at least 3 cards in each of the unbid suits. Your partner must bid unless the opposition intervene.
. . .you have a strong hand (16+ points), no 5 card major and 2 stops in opposition's suit. If not use a takeout double.
Use a jump overcall if you have a weak hand and at least 6 cards and 2 honours. This very descriptive preemptive bid disrupts opposition's bidding whilst giving the opportunity of a sacrifice contract if partner has a fit. A sacrifice contract is only worthwhile if opposition are vulnerable and you are not.
Overcalling in Spades is especially disruptive because it forces the opposition to a higher level.
With an Ace and King in a 5 card suit but little chance of competing in the auction, an overcall can give a lead indicator for the defence. The opening lead in defence can defeat the contract. SO lead partners suit so the overcall is justified.
Tread carefully! Partner might not have an opening hand.
With 3 cards in the overcaller's suit, support according to your strength.
With no support and fewer than opening points - pass.
Only change suits if you have fewer than 2 in overcaller's suit and 6 cards of reasonable quality in your new suit.
With <3 card support but an opening hand, bid oppositions suit at the 2 level - a Cue bid. With a weak hand (<10 points) the overcaller will respond to a cue bid by bidding 2NT. With an opening hand, the overcaller will invite game by bidding 3 of his suit or bidding a second suit.