Is it necessary to have the instruments networked together? No. It is nice? Absolutely.
Are your Raymarine instruments networked to your Autopilot? IE do you have two separate networks, one for the Raymarine gear and another for the Lowrance? Or perhaps the Lowrance isn't connected to anything else? Either way, here are some advantages of having instruments, Plotter, Autopilot, VHF all communicating:
True / Apparent wind:
With standalone instruments maybe you can get true and apparent wind, maybe no. With networked instruments you DO have true and apparent, and you can display that information on the plotter, and send that information to the autopilot. This allows the pilot to steer to a wind angle. Personally I like this function and use it frequently if I am just out for a few hours sail with no real destination. I pick a wind angle (usually 60 degrees apparent), engage the AP on that wind angle, trim the sails, then relax. I can do anything I want and the boat will stay on the wind angle and the sails properly trimmed.
Instrument data display on plotter:
My instrument displays are mounted on the bulkhead next to the companionway (the previous owner mounted them there). If I have guests aboard they frequently will sit where the instrument display is blocked. Since I can display any information I want on my plotter (Vulcan 7) I don't need to constantly tell my guests to move. Of course if I'm racing the crew knows to stay out the way, but for sailing with guests I want to nice to them (most of the time).
Navigating to a destination:
As previously described, the plotter can provide instructions to the AP. This can be for a single waypoint or many waypoints along a route. This is nice if I'm doing a long delivery.
All current VHF radios include DSC. The radio must be GPS information for DSC to work. There many ways to accomplish this, including VHF w onboard GPS. A VHF w NMEA 2000 networking makes this super easy. Many new VHF radios include an AIS receiver as well. My Simrad RS35 does and I can see AIS targets on my plotter, including information on range, bearing, speed, CPA, etc.
If it were difficult or expensive to network different units together I would not do it. However, since Raymarine has the Seatalk to Seatalk NG kit available, all you need is the kit and some cables. For under $100 you can have all of the instruments networked.
I have not felt a need to try to interface it with anything. We also have a late model Lowrance hi-def radar and 9" plotter at the helm. Wonderful stuff, especially the radar with its ability to resolve small targets.
Our older Raymarine ST-60 instrument cluster is on its own separate network.
It would be fun to tie this stuff together someday with a 'black box' interface, but there's just no need to do so.