The answer to this question can be found in Act IV scene 3 of this excellent tragedy, just after Brutus has heard the news of his wife's death. When Brutus suggests that they meet Antony's forces at Philippi, it is interesting that Cassius disagrees, arguing that it is better for the enemy to seek them rather than them seeking the enemy.
However, Brutus gives two reasons for why they should meet the enemy at Philippi now. Firstly, the people between Philippi and where they are now support their forces only grudgingly. Brutus fears that the enemy would gain extra recruits from them if they marched through their land. Meeting them at Philippi would prevent this advantage to the enemy.
Secondly, Brutus thinks that his army is at the strongest point it can hope to be, therefore it is much better for it to fight now rather than wait for morale and strength to decline. Note what he says:
We, at the height, are ready to decline.
There is a tide in the affairs of men
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
On such a full sea are we now afloat,
And we must take the current when it serves,
Or lose our ventures.
Brutus thus argues that he and his troops must seize the day and fight now when they are at their highest level in terms of strength, courage and bravery, making the most of "the current when it serves."