It is free, applied for directly with the IRS, and usually issued immediately online. You need one if you form a corporation or a multi-member LLC, if you hire employees, or if you file certain excise or employment tax returns. Many single-member LLCs and sole proprietors are not strictly required to have one but get one anyway, because banks, payment processors and clients routinely ask for it.
The practical reason to get an EIN early is separation. Using it instead of your Social Security number on W-9s, bank applications and vendor forms keeps your personal identifier out of circulation and reinforces the boundary between you and the business — the same boundary that limited liability depends on. Each separate legal entity needs its own EIN. If you operate three LLCs, that is three numbers, three bank accounts and three sets of filings, and blending them is one of the fastest ways to weaken the protection you formed them for.
A concrete example: you form a single-member LLC for a consulting practice. Technically you could operate on your SSN. Instead you apply for an EIN in about ten minutes, open a business bank account with it, and put it on every client W-9. When you later form a second LLC for an unrelated product, it gets its own EIN and its own account — so when tax season arrives, the two businesses' records were never entangled in the first place.