The number one reason to GPU mine cryptocurrency is the simply fact that GPU hashing and compute can be pointed at dozens or even hundreds of different coins whereas ASICs are locked to a single mining algo. High end custom ASIC mining hardware is typically built for a single altcoin algo. For example, Bitmain’s Antminer line is purpose built to mine SHA256 based coins like Bitcoin (BTC). Innosilicon’s A6+ LTC Master miner is designed to mine Scrypt algorythem based coins like Litecoin (LTC), Dogecoin (DOGE) and Mincoin (MNC). These miners can only mine a single algorithm and while there are many coins to choose from, you are still limited to that specific algo.
With GPU mining, you can mine almost any coin. That means as profitable changes, for example, you can point your GPU hash to a new coins. Many miners, especially smaller retail miners, prefer this degree of flexibility. The primary trade-off with GPU mining is that its almost always less efficient than ASIC mining. The vast majority of large scale commercial mining operations are using ASIC miners for their hash. When you are mining altcoins at scale with 1000s of miners in a facility, every single penny counts. In conclusion, if you are a beginner miner or small retail investor with a small budget, using a video card GPU makes sense. You can even use an old card from a used computer to start out with. However, if you are a serious player or mining operation, the only solution that makes financial sense is to use ASIC mining equipment.