Without understanding the semantics of such features, you may also have difficulty discussing programming language ideas with others, and will have difficulty in reading the technical literature. If you are planning in specializing in some other area of computer science, you may someday need to read some of the literature on programming languages, either to use results from programming languages, or to apply ideas from your area to programming language research. Knowing how to solve problems using the different paradigms is important for several reasons. You can find solutions to problems more surely if you have many different ways to approach problems. Already the use of functional languages is increasing, and the large telephone company Ericsson uses the functional language Erlang to write all of its software.
Similar comments hold true of the object-oriented paradigm.
Type declarations are a simple form of formal specification, and type checking is a simple, automatic form of program verification. Type checking is believed to be of great help in programming, because it catches errors before a program is run, and type information is used heavily in optimizing compilers to improve generated code. These techniques are a hot topic of research, and have been so for years. Studying type systems and paying attention to type issues in language design seems to help organize and regularize a language design. The study of semantic interpreters and formal semantic description techniques reinforce each other and, we believe, help solidify your understanding of major programming language features.