To be honest, people aren't saving a huge amount with the tax-free staus that comes with an ISA, but it all helps.
If you've managed to stash away £50k in an ISA and currently get 3.0%, then the gross interest on that would be £1500 each year.
If you're a 20% tax payer and had to pay tax on this, your net amount would be £1200 and if you're a 40% tax payer, your net amount would be £900.
Savings of £300 a year, for a basic rate tax payer and £600 a year, for higher rate tax payers are not exactly huge amounts in my opinion.