ContractorTools provides a lot of construction specific business functions, and you can certainly use it without syncing to QuickBooks, but there are many functions that QuickBooks provides that ContractorTools does not (and does not plan to) provide.
A good way to look at this is that ContractorTools provides functions that are specific to the construction business, while QuickBooks provides general accounting functions are needed by all types of small businesses. These include:
Accounts Receivable (AR) is a means of keeping track of customers who owe you for invoices that you have submitted to them, the amounts that they owe, and whether the amounts are past due, and how long they are past due. AR functions also include sending statements to customers
Accounts Payable (AP) is a system that keeps track of who you owe money to, how much you owe, when it is due, or how long it is overdue.
General Ledger (GL) accounting is a system that records and reports on all of the financial transactions in a business. The information in the GL system is used to provide important reports such as Profit & Loss and your Balance Sheet. The Profit & Loss report shows how much money you made or lost in a given period of time. Your balance sheet is a financial statement that reports the “book value” of your business, as calculated by subtracting all of the company's liabilities and owner equity from its total assets.
The single largest source of financial transaction data needed for General Ledger accounting is your bank account (income and expenses) activity. QuickBooks makes it very easy to connect to your bank account and download all transaction activity from your bank account. All of this information is needed to perform Accounts Receivable, Accounts Payable, General Ledger, and Tax accounting. If you don't use QuickBooks, all of this information will have to be recorded and tracked somehow.
The information provided in the Profit & Loss report is needed in order to prepare your income tax returns. It is very easy and efficient to record all of your financial transactions in QuickBooks and then click a button to print out your Profit & Loss report to give to your tax preparer at the end of the year. Without that, you will need to use some other means of keeping track of all of your financial transactions (a shoe box?) and provide them to your accountant in order for them to prepare your tax returns. If you don't use QuickBooks to do this, you will have to pay your accountant to manually enter all of that information (into QuickBooks), and the cost to have them do that is much more than the cost of an annual subscription to QuickBooks.
If you ever want to obtain a business loan, your lending institution will require that you provide both a Profit & Loss report as well as a Balance Sheet. It is very easy and efficient to record all of your financial transactions in QuickBooks and then click a button to print out your Profit & Loss and Balance Sheet reports to provide to your lender. If you do not use QuickBooks, you will need to scrape up all of this information and assemble it manually. The cost to pay a bookkeeper or accountant to do that is much more than the cost of an annual subscription to QuickBooks.