They're just locking in the myth that tax revenue funds expenditure.
If you inherited a company that was struggling because of lack of investment you would not expect to fund the investment needed to bring it back to profitability out of its meagre profits (it's a struggling company, don't forget). There wouldn't be enough money to do it. You would borrow to invest in measures that would restore it to health.
Governments which run a sovereign currency don't even need to borrow, they can 'create' money to invest in the country. That's where £240+ billion of quantitative easing came from after the bankers' crash, the fall in the pound after the EU Referendum and May's £1 billion bribe to the DUP. Most of the invested money comes back to the government by way of tax.
The Lib Dem's policy locks us into only spending what we have. As a significant amount of this will go abroad through the purchase of resources from other countries, or into offshore tax havens as some companies and individuals squirrel away their profits where the tax man can't get hold of them, or into personal savings, then the available supply of money from tax revenue diminishes, thus (if you really want to believe that taxation funds spending) leaving less and less available for state spending.
Result? Cuts and austerity...