In many collision cases mariners do not reduce speed to avoid it. This happens as they are either in a hurry to attain sea speed after dropping the outbound pilot or they hesitate to touch the telegraph to reduce the speed when navigating in areas with high traffic density.
There could be many reasons for this hesitation, like commercial pressures to keep ETA the same, tides to catch, worries that engine would sustain damage, penalties on late arrival, and maintaining performance of the ship in relation to speed-consumption.
However, Gard informs that despite whatever reason to not reduce the speed, this measures can have many benefits. Namely, it can give the operator more time to assess the situation, but also if the collision does take place, its damage would not be as severe to both the vessels.
Finally, hard helm orders without reducing speed could lead to large over-swings which can become difficult to control.