Similar trademarks can be harmful to your application, either because an examiner will cite them during examination or they'll oppose your mark's registration.
During examination, the IP office may refuse your application on grounds of the likelihood of confusion with an earlier mark. This is more common in some jurisdictions than others. The EUIPO, for example, does not raise relative grounds on its own initiative, whereas some national offices do.
Furthermore, during the opposition period, the owner of the earlier mark can file an opposition, temporarily halting your registration. If the opposition succeeds, your application is refused. Even if it fails, opposition proceedings add time and cost to the process.
The risk of both can be assessed before filing with a trademark search. If a conflicting mark is identified early, you can adjust your application or approach the other party before proceedings begin.
A free trademark search with Trama can identify conflicting marks before you file.