Why put guns on a Capital ship?

The primary tactic of a Capital ship is not to close with the enemy, but to stay back behind its fleet and deal death, preferably to opposing Capital ships.  Kinetic weapons have quite short ranges, so cannot fire until enemy ships close in.

[Look!  The ghost of World of Warships!]

Regarding tactical speed, I am unaware of it having a tactical difference due to the "simulator" simply driving the ships towards each other. It either takes a little longer or a little less longer. As it relates to jamming, that is a different story. As far as I know, the only real reason to put tactical speed components on is to try to stack jamming. As far as escort speed vs capital speed, the escorts will loiter around the capitals so I would think their speed is only an issue if they cannot keep up with the capitals charging at the enemy.

In regards to Capital ships with guns, I look at that as simply the biggest concentration of firepower possible (think old wooden ships-of-the-line with 40+ cannons on multiple decks). Yeah, they do have to drift up a bit, but once they get there the knockout punch arrives. I also keep my fleet compositions very simple, either escort medium hulls with large capital hulls or total "support" fleets (given that a lot of the designations don't make much sense to me at the moment).