As a California guy....uh....???
I'm joking. The manual says to avoid the old school chains. So look for the new bead style ones. I think they are call snow cables?
As for when you need them...Thats a question for some one that actually lives in snowy areas. My only experience is when I go up to Tahoe and the road signs say to use chains.

The other option and more $$ is to buy winter/snow tires/studded tires.
I'm sure others would chime in later.

 

Depends what you are going to do. The later Foresters can handle quite a bit. Your generation may not be quite as good.

 

With the right tires, the SJ series acquits itself quite well in snow... I've had mine in snow using "lowly" all-season tread without the 3-peaks designation. And not really any issues. Sure, trying to peg-the-pedal from a stop is not a grand idea, but for most "normal" driving, your Forester should be fine.

As mentioned by some, depending on where you live, an investment in a 2nd set of wheels with dedicated snow tires is not a bad idea if you life in an area and you have to drive daily in snow.

And yes, traditional snow "chains" will not work on the Forester. You need to buy the cables and/or strap type devices.

 

Get a set of 3 Peak rated tires and you will never need to put "chains" on, although you may need to show them to one of the boys in blue.
With the low profile of the Subaru wheel well, snow cables are okay and chains aren't, but again, with a good set of tires, either would just be for show.

 

Where do you live?

If you drive in certain places in the Pacific Northwest, chains can be mandatory in certain situations.

And,

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Carrying chains in the PacWest is a requirement for higher elevations.
Putting them on almost/never happens if you have snow rated tires.
In my experience, if you really NEED to put cables on a Subaru, you shouldn't be out on the road.