In numbers: if a complete process costs $100K and the investment is 2M the fund is out 2.1M. If they pay the bills from the fund directly they're out 2.1M without a sales tax return, if they pay 2.1 to the company and the company pays the bills the company gets back the sales tax so effectively this saves 20K or in this example about 1%. Not much but also not nothing, and if a fund does a lot of smaller deals this sort of thing adds up.
Not all funds suffer from this (likely Spark isn't one of them) so the ability of other funds to match this is dependent on whether or not they have classed themselves in such a way that they are sales tax exempt.
I'd imagine Spark to be able to offer exactly the legal fees less for the same amount of stock in company 'x' compared to some competitor that does it the usual way so effectively this shouldn't matter but it is excellent publicity for Spark.