Probably the biggest problem most startups have is getting exposure for their product. Often times they think they just need to put something on the internet and the buyers/users will come and nothing could be further from the truth.
It is sometimes better to do a partial development and get some initial feedback from the market and see if there truly is interest in the product. If there is, then it's worth investing more money to finish everything up; if not, then your losses should be significantly less than were you to go all the way to the end, finish it up and find that the market isn't responding.
It's a balancing act of figuring out how much you need to develop to be able to gauge consumer demand for the product so if you can develop something without a lot of out of pocket expenses then go ahead and develop but if it's something that is intensive and costly, best to stick your toe in the water and see how the water is before committing completely.
Better to find out something isn't going to work for a little money than have all your hopes set on something that ultimately isn't going to see the light of day in the market.
Another thing to consider is that if you develop a product through to the end and discover that it needs to be changed, then it will be more expense to make changes. On the other hand, a slow and partial development can yield great insights into the market and allow you the opportunity to "pivot" and change gears. The allows you to finalize the development more to what the market will bear.
In my opinion, you never develop all the way to the end because the consumer hasn't yet weighed in on whether the product is what it ultimately needs to be to yield the greatest success.