We are based in UAE. Home currency is AED. However, when I try to link company bank account wit QB I get a message to switch on Multicurrency? WHY is this mandatory?

Thanks for coming in the Community, @LYMaccounts. 

Can you clarify if it's a foreign account that you're trying to connect to? If you have bank accounts that use a currency other than your local currency, you'll have to turn on Multicurrency in your Advanced settings to properly track these transactions, as well as exchange rate gains and losses. Once you turn this feature on, you unable to turn it off again.

Here's how:

1. Go to the Gear icon.
2. Choose Account and Settings.
3. Select the Advanced tab.
4. Click Edit ✎ in the Currency section and choose your Home Currency.
5. Select the Multicurrency checkbox and confirm that you understand that you can't undo this option.
   
6. Select Save and close.

Then, follow these instructions to add an account that uses a foreign currency:

1. Go to the Gear icon.
2. Select Chart of Accounts.
3. Select New.
4. In the Account dialog, select the appropriate Account Type and Detail Type.
5. Enter a Name for the new account and an optional Description.
6. Select the currency to assign to the account from the Currency ▼ drop-down menu.
7. Select Save and Close

Once done, you can connect your bank account to QuickBooks Online. 

I've also got these helpful articles for future reference:

• Turn on and use multicurrency
• Assign a foreign currency or multicurrency to a customer

Let me know should you need further assistance in categorizing and matching online bank transactions in QuickBooks Online. It's my priority that you find the success you need.