I use the Baggs Para DI with my D-55 when I play at church. The Baggs unit is a separate, stand-alone, combination preamp / parametric EQ / Direct Injection (DI) box. You plug your guitar cable into that, then plug the box into a PA or amp.
I have a K&K passive pickup in my D-55, so I need an external preamp if I want my own control of EQ settings and volume. If you already have a preamp in your guitar, like the Fishman in your F47 (and the Baggs unit in your soon to be F47R), you dont need the additional preamp function.
A parametric EQ is more sophisticated than typical bass, mids, and treble preamp controls 'cause it lets you isolate a narrow range of frequencies for gain boost or reduction. The Baggs Para DI is actually a semi-parametric EQ. Not as functional as a full parametric EQ, but more than just bass and treble.
You also have a semi-parametric EQ in the Fishman unit on your F47, albeit more limited. The Frequency slider sets the frequency, and the Contour slider sets the boost or cut.
The Baggs Para DI gives about the same control, but identifies the frequency by number.
The DI, direct injection, portion lets you plug the guitar directly into a PA system. Hense, direct inection. It takes the unbalanced signal from the guitar 1/4-inch jack and converts it to a balanced signal out through a standard microphone jack (XLR) to the PA system. In a balanced signal, there is negligible signal loss in long cables. If you just ran a long guitar (instrument) cable with an unbalanced signal for a long distance, there would be signal loss.
So in summary, the Para DI would give marginally more EQ control than you have now, and the ability to plug directly into a PA. Although I think your Marshall has a line-out jack that would also go directly into a PA.
Do you need it? No
If you were playing out, you might find it usefuil if you could not get the sound you want with your on-guitar controls and your Marshall amp, or if you had feedback problems on stage.