Why pay for an art valuation when I can get a free valuation elsewhere?
Of course it's entirely up to you whether you use a paid for, or a free art valuation service.
However, it's worth bearing in mind that free valuations are usually provided as an incentive to use follow-on services.
A free art valuation may be offered with the intention of getting to know what artworks you possess, to encourage you to sell, or to generate business for the 'free' valuer when they offer to buy or sell your artwork for you.
Free valuations can be quick approximations of value designed to facilitate a bargain buy, or to encourage a transaction. Such valuations tend to be on the low side, and while free, may prove to be a false economy in the long run.
You may therefore prefer to pay a modest fee for a stand-alone art valuation, rather than accept a free valuation which may only be offered as an incentive to use other services.