A day tank does come in handy when you have more than one tank or some boats have even more. It can simplify the plumbing
of supply and return lines and make bleeding the system easier. It means each tank only needs a vent, fill, and a supply line but no return line. The day tank would then be the only tank with a return line. The process of switching between tanks
can be a little more complex.
You then have to transfer fuel
into the day tank and can choose which tank to pull it from and hopefully balance the boat more.
Any fuel pickup can suck air. A tank near empty can do it easier should the fuel slosh around. A well positioned day tank can make it harder since the tank is smaller.
As far as gravity flow I don't see that as a huge advantage you still use a lift
pump and the line still must be bled.
Our boat has two tanks
and the process of switching tanks requires a time consuming process and manually flipping several valves that are not easy to quickly. I've thought about eliminating the dual plumbing
and just adding a transfer pump to move fuel from tank 2 to tank 1. The trade
off is then I have to manually make sure I don't over fill tank 1.
Our prior boat had two tanks and a nice switch mechanism to switch over. Just flip two valves that were easy to get at. One 30 gallon tank was outboard
and the other 30 gallon tank was mid line. I did on the very first cruise
trip suck air out of the outboard
tank because the tank was only about 1/8 full and being outboard in some very nasty weather
is sloshed the fuel and sucked air.
If I had only one tank I don't see why a day tank would help except for one application. If you have a diesel heater you can mount a day tank high enough that you don't require a fuel pump
or a return line. It does not work that way for an engine though.