Why were the British marching to Concord?
Their mission was to capture the guns and gun powder stored in Concord.
Starting around 1763 the King and parliament passed a series of measures aimed at reeling in what they saw as an out of control colony in America. One such act was aimed at dissolving the American militia by making it illegal to keep large stores of guns and gun powder.
Gen. Gage made it his mission during 1774 and 1775 to confiscate all such known stores. He made unsuccessful forays to Portsmouth NH, Salem MA, and Plymouth MA with small company sized military. That meant roughly 100 men. When he set out after the munitions in Concord he sent nearly 900 men to capture the weapons stored there.