Brest was the French Atlantic base whose fleet could threaten the Channel once it escaped. Blockading the port and watching the routes that could connect other French forces with it became a recurring British strategic problem. The Western Squadron consequently had a larger role than coastal patrol: it had to prevent an operational French concentration. Battles such as Trafalgar fit within this wider system because a successful Mediterranean fleet still needed to join or release the force at Brest before it could create the greatest danger to Britain.
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Why Brest made the Western Squadron indispensable
Brest's role as France's key Atlantic fleet base explains why the Western Squadron became a central British force.