The most straightforward reason why the stock price jumps is because the acquirer generally makes a bid at a premium to the potential acquiree's stock value in order for the deal to make sense.
Market participants know this, especially professional traders, so when the news comes out the fastest players will hop on the stock to make a quick profit. Those who get in on it the fastest when the news breaks enjoy an increase of several percent (or sometimes significantly higher, such as in Linkedin's case!).
The long term reasons and implications are more complex, but from a short term game theoretic perspective if you can quickly act on the knowledge of an acquisition, you can make quite a bit of money. Most people don't have the savvy or infrastructure in place to do this, but if you're drinking coffee in the morning and you see this pop up on your Bloomberg terminal, you'll want to buy within seconds/minutes if you believe the stock is going to jump. You can go so far as to automate the process and place orders for the stock as soon as an algorithm recognizes an acquisition announcement.
There is also the phenomenon where you can reasonably assume that M&A announcements were leaked in hindsight by looking at the option trading history for the stock. Very often you see that in the week or days prior to an announcement, extraordinarily lucky "bets" are placed in the form of call options, indicating that someone knew and acted on the information. Matt Levine has a great Bloomsberg column article on this with several examples.