There were two reasons why General Thomas Gage (who was the governor of Massachusetts at the time) sent British troops to Lexington and Concord in April of 1775. Both had to do with the rebellion that was brewing in the colony at the time.
First, Gage had heard that the Patriots had been collecting weapons to use in a potential rebellion. The Patriots were said to have stored the weapons in Concord. Gage sent troops to Concord to find and confiscate the weapons.
Second, Gage felt that he would be able to capture some Patriot leaders in this way. He had heard that John Hancock and Samuel Adams were hiding in Concord as well.
Gage sent the troops to Concord for these two reasons. Lexington was simply a town along the way to Concord.
700 British Army Regulars under the command of Lieutenant Colonel Francis Smith were sent by General Gage on order of the crown for the purposes of:
1) Seizing, capturing, and destroying all stores of ammunition, powder, and any arms discoverd, including individual arms belonging to private British Subjects, ... and ...
2) To arrest John Hancock and Samuel Adams. While locals viewed them as Patriots and Champions of Liberty, the crown considered them traitors.