Direct Debit You (the business) control the collection. You set the amount, the date, and the frequency. Your customer authorises it once by signing a mandate, and you collect from there. Payments can vary in amount. Protected by the Direct Debit Guarantee.
Standing order Your customer sets this up themselves through their bank. They control the amount and frequency. If you need to change the amount, the customer has to update it manually. No guarantee protection. If they forget to update it or cancel it, you don't get paid.
Bank transfer A one off manual payment initiated by the customer. They need to log into their bank, enter your details, and send the money every time. No automation. No guarantee. Easy to forget or delay.
In short
Direct Debit gives you the control. You decide when to collect and how much. Your customers don't have to remember anything, and you don't have to chase. Standing orders and bank transfers rely on your customer taking action every single time, which means late payments, missed payments, and unnecessary admin for both sides.