A police officer may arrest somebody accused of an offence                      
I.        To prevent such person from committing any further offence                    
II.        For proper investigation of the case                   
III.        To prevent tampering of evidence

Section 41 CrPC When police may arrest without warrant.—(1) Any police officer may without an order from a Magistrate and without a warrant, arrest any person— (a) who commits, in the presence of a police officer, a cognizable offence; (b) against whom a reasonable complaint has been made, or credible information has been received, or a reasonable suspicion exists that he has committed a cognizable offence punishable with imprisonment for a term which may be less than seven years or which may extend to seven years whether with or without fine, if the following conditions are satisfied, namely:— (i) the police officer has reason to believe on the basis of such complaint, information, or suspicion that such person has committed the said offence; (ii) the police officer is satisfied that such arrest is necessary— (a) to prevent such person from committing any further offence; or (b) for proper investigation of the offence; or (c) to prevent such person from causing the evidence of the offence to disappear or tampering with such evidence in any manner; or (d) to prevent such person from making any inducement, threat or promise to any person acquainted with the facts of the case so as to dissuade him from disclosing such facts to the Court or to the police officer; or (e) as unless such person is arrested, his presence in the Court whenever required cannot be ensured, and the police officer shall record while making such arrest, his reasons in writing