They are an expensive way of buying salts in easy to use premeasured doses.
Table salt would be fine for most. Epsom salts, andrews liver salts and Lo salt if you really want pottassium and magnesium as well.
There are two issues that often get mixed up.
1) You lose salts thru sweat and these need to be replaced. You can do this by ingesting the salts in many ways from using electrolyte products to eating crisps
2) your body absorbs water best from an isotonic solution. For salt replacement isotonic is simply unnecessary. For fluid replacement it is best but not essential it is isotonic.
Be careful and check the ingredients. If you are not carefull you can either end up with too much salt or a huge glucose load depending on the type of product.
Myself I far prefer a more natural approach. 1/3 litre apple juice to 2/3 litre water with a couple of teaspoons of table sugar and a stingy teaspoonful of salt. I prefer a slightly hypotonic solution
I am no elite athelete tho