I'm the guilty party here,as I am the principal offender when it comes to adding ships to BAT!
The reason is exactly as you guessed - to make compiling task forces easier in missions.
To summarise: all CV/CVL and ships that would typically comprise a carrier task force (BB, CA, CL, DD) are set to this default speed (25 knots). This gives you enough speed for non-catapult takeoff generally with no messing about with wind speed/direction in FMB.
Major warships that are not required to support carriers, principally German and Italian, are set to 18 knots default.
Merchants that would form part of a convoy are set to 12 knots, as are obvious convoy escorts e.g. Flowers.
Submarines I never got around to fully standardising, as they are not typically going to be combined with other vessels, so it does not matter much. However submerged are 4 knots, surfaced around 9 knots.
Sailing vessels are 5 knots.
Obviously these can always be adjusted on a case-by-case basis when designing a specific mission if appropriate. Note that the default is 50% of full speed in the game, so technically (but not realistically) you could have your fleet carriers travelling at 50 knots!
Yours,
Clive