I am having an old Dell Inspiron 1525 laptop. Recently I bought myself a Bose QC15 headphone. I find the sound I get on the headphone a bit thin (lacks depth). I mostly listen to FLACs. I was wondering if an external sound card like this can bring any considerable difference to my music listening experience. I am not intending to connect to speakers.Could you please comment on whether I do need external card.
If you are listening to .flac files for the same reasons as me (maximum possible sound quality) you will always encounter quality problems with notebook or USB soundcards. I can only suggest you look at the Creative store (maybe at a product like this). I found the Creative sound products to be really satisfying, exspecially for their low pricing. I've not tried their USB sound cards, but I own a ZEN X-FI II and love its sound.
Internal sound cards often let you hear noise that comes from imperfect shielding. This is especially audible with good headphones (and bad hardware, of course).
So, yes, the only way to overcome this is to buy an external sound card. I can't tell you how good the one you linked to is, but I'm sure it will deliver better quality than the internal one.
The best way would be to buy an Audio Interface that can also be used for recording, because they have dedicated headphone amplifiers. In the end, the signal chain matters, and mostly "normal" sound cards only have low budget headphone amplifiers. There are also standalone headphone amplifiers available, you just have to google for that.