The idea behind adding lime as a soil amendment is to neutralize excessive acid. For example, if you want to use peat moss, it's below the range you want. So you add lime to bring it up to 6.8.
It also buffers the effect of chem fertilizer which can acidifyu the soil over time. But don't use too much if that's the only reason you're adding it; remember, pH 6.3 to 6.8 for soil, up to 7.0 since the pH will drop a bit with fert use, esp. if you aren't adjusting the water.