Dissolved minerals can affect developer activity, among other things, but the real problem is that in most areas with hard water the mineral content varies with the seasons, the temperature, and with how the local water authority routes the water through it's domain.

Photographic chemistry is not about being accurate, but it is about being consistent. If your water supply has a constant mineral content you can adjust your processing and exposure to compensate for any effects it may have. With a varying mineral content results can become unpredictable, and unless you like re-shooting negatives it makes sense to eliminate unpredictability, particularly if the cost is low as in this case.

Bottled water often has a high mineral content (it's part of the sales pitch). Expensive brands like Evian work hard to keep it constant, but cheap ones vary just like your tap water so distilled or deionised water makes more sense. I don't know about Isreal, but in most of Europe it can be bought from pharmacies in multi-gallon sizes for not much money.