Wikipedia article about North Atlantic Tracks says:

They are aligned in such a way as to minimize any head winds and maximize tail winds impact on the aircraft. This results in much more efficiency by reducing fuel burn and flight time. To make such efficiencies possible, the routes are created twice daily to take account of the shifting of the winds aloft and the principal traffic flow

Article links to daily map of tracks, complete with the position of the jetstream.

Wikipedia article about North Atlantic Tracks says it is to avoid jet stream winds (when flying west) or use it (when flying east):

They are aligned in such a way as to minimize any head winds and maximize tail winds impact on the aircraft. This results in much more efficiency by reducing fuel burn and flight time. To make such efficiencies possible, the routes are created twice daily to take account of the shifting of the winds aloft and the principal traffic flow

Article links to daily map of tracks, complete with the position of the jet stream.

Jet stream winds have average speed 100 mph (160 km/h) so they are significant.