It's not always economical - or safe - to use the strongest nic-juice you can find. You will almost always get by with nic-juice that is 1.5 times stronger than the e-juice you vape. If you vape 16mg, then 24mg will generally be sufficient for DIY mixing and will save you some cash compared to mixing with a stronger nic-juice from the same company.
This "1.5 times" number allows you to have e-juice with 1/3 of it being flavoring and other ingredients. If you use weak flavorings, or like to add VG to get vapor production, then you might want a stronger nic-juice.
Making a 30ml bottle of 24mg e-juice using Blip's weakest nic-juice (38mg) will cost you $15.41, but making the same exact e-juice using their 75mg will cost you $20.76. Strangely enough, their mid-level 56mg will bring the price of that bottle to $21.45.
You can save quite a bit of money by buying nic-juice at the appropriate level. Of course, if you're just starting out then you might not really know what the appropriate level is, so buying the strongest nic-juice allows you make e-juice at most any (reasonable) nicotine strength.