Does a silo need to be powered to provide storage for raw materials and items?

No, but helps. Only have to worry about incomed storage facils; hangers, warehouses, landingpads.. ect.

For what reason does it help?

It doesn't. As far as I know power for silos has no real effect. Might possibly mean that its flats/jobs actually count (also for other facilities).

If it is underpowered it likely means that its affect on the employment rate will end, which in the long run could affect the income of the city. But since silos are very small, it would have a minimal effect.

in a similar vein, does a warehouse need to be powered to store stuff? Only to store stuff, for income I know it needs to be powered.

Any facility that is unpowered, at the moment, can be entered and walked around and stuff left in there. Vehicles and ships too can be docked in facilities that have the right abilities with no power.

All that will happen if they don't have power is that their effect on morale/crime/ER will not take place, and you cannot use their abilities (production, mining etc).

“if they don't have power is that their effect on morale/crime/ER will not take place ”

You sure about this? Since when?

Since never.

Nah..once a facil finishes,powered or not,it affects all planet stats including MNSF.Only way I know to manipulate MNSF is closing crewlists.

It used to be an unpowered facil was un-enterable..period.No power for the door or something

The difference between a powered and unpowered silo/warehouse is that if they are owned by a mining faction with an active mine in the same city, and the mine's capacity is reached, then the automatic RM transferal happens only to the powered silos/warehouses. With the unpowered ones you would need to manually move those RMs from the mine.

That shouldn't be happening with warehouses, just silos.