Is there any example for real project where field of deciamal type been used?
Here's a link to an explanation of Drupal's different number types (integer vs. decimal vs. float) with some examples. Basically, it breaks down as follows:


Integer: You're storing numbers without fractional digits (e.g. a field that stores a product stock quantity, XX units)
Decimal: You're storing numbers with a fixed amount of fractional digits. (e.g. a field that stores a price, $XXX.YY) 
Float: You're storing numbers with a variable amount of fractional digits. (e.g. a field that stores latitude/longitude coordinates, XX.XXXXXXX..., YY.YYYYY...)


Also of note is the underlying data structures of integer/decimal/float that gets stored in the database. See Fixed point vs. Floating point for some discussion on the differences.