The benefits are related to protecting the various household appliances that are fed by your water supply. Hard water leads to heavy limescale deposits which can damage your heating system. The most sensitive item would be the boiler for hot water and central heating. This leads to many people believing that there is an environmental benefit to softening the water (e.g. Kind Water in the UK), but it is hard to find hard figures behind the claims.
In the US there are concerns about the environmental impact of softened water, which, when the softening is done by a typical household appliance, contains significant levels of sodium (A Hard Look at Water Softeners). The discharge of this sodium in waste water causes damage outside the house -- this is a greater problem that the water additional water usage which you mention.
The salt consumption you mention appears to match the expected usage for a 4-person household (How Much Salt Should my Water Softener Use?). On the energy use, it is often argued that the energy saved through extending boiler life-time is much greater than the energy used (e.g. Home Water 101), but such advocacy tends to ignore the environmental issues caused by salty waste water.
The environmental impact of salty waste water will depend on your location: if your water discharges into the ocean, the impact is zero, but if you are in an arid inland area the impact may be significant.
Unfortunately, there does not appear to be a good alternative for people living in hard water areas where salt discharge is a problem. In a typical household system, all the water apart from drinking water is softened. If you can set it up so that you only soften the water going through the boiler, this could reduce the negative side of the balance (e.g. softening water that then flushes a toilet is pure waste) -- but this is likely to be complicated. For more information you could probably consult your local water authority or conservation groups.
You can use potassium salts instead of sodium salts to reduce the negative environmental impact, but this is significantly more expensive.