As you know, in Islam we are prohibited using interest for returns. Thus, in conventional bank they are using interest to give return for depositors. More over, they also invest our money in any institutions. Some of our money they might invest in casino, beer company etc which is not Halal for the income of Muslim. More over it can give higher returns of investment for the bank.
In Islamic institutions such as bank, they will invest our money only in Halal institutions only and giving dividend as returns to the depositor instead interest which is not allowed by Islam. The rate of return dividend doesn’t fix like interest. However, the return sometime can be increase or decrease. Thus, the return sometimes can be higher compared to the conventional institutions. Most of the time the return is high compared to conventional institutions (interest).
If you own conventional saving account, why not you just change your account to Al-Wadiah / Al-mudharabah since the return would be higher. More over, the return is halal!