What is the point in publishing a paper in a journal rather than arXiv? I'm undergraduate student in Physics, I wonder what is the point in publishing a paper in a journal rather than arXiv ? 

As far as I know, there is a fee which must be paid for publishing in a journal (It may not be valid for all journal or fields, I guess).I'm asking because If I do some publishable work on my undergrad research, what option would be more logical and advantageous ? How would they affect my future Phd admissions ?

Or if I ask my second question more generally, How would these options affect one's carrier in academia ?
Papers published in (reputable) journals are reviewed by other scientists (peer review), which usually makes it considerably more difficult to publish a paper there. By contrast papers on the ArXiv only receive a brief inspection to keep out utter crap. Thus, most academic evaluations consider only peer-reviewed publications or value them considerably higher.

Moreover, often only peer-reviewed papers are considered citable, in particular for purposes of backing up your claims. This may extend to papers that are considered certain to be eventually peer-reviewed¹, but this is unlikely to apply to your paper. As being cited (by peer-reviewed papers) is another important academic evaluation metric, this is another advantageous aspect of journal publications.

As a sidenote: There are a lot of physics journals where you can publish without a fee.



¹ in particular in fast-moving fields or fields with long peer review such as parts of mathematics