To your question,

So why do i need to make periodical transaction log backups (DBAs

suggest every 15 mins after full backup), if i can restore to any point in time after full backup without making any periodical transaction log backups.

You can restore to that point in time ( in your case the last full backup as you do not have any T-log backups). So, if you do not have any T-log backups then you can restore only to that point ( last full back ) and you lost all the valuable data from that point. I think you are smart enough to know the value of data. :)